# Chaitanya's Random Pages

## December 30, 2020

### An identity involving the product of reciprocals

Filed under: mathematics — ckrao @ 10:18 am

In this post I would like to prove the following identity, motivated by this tweet.

$\displaystyle n! \prod_{k=0}^n \frac{1}{x+k} = \frac{1}{x\binom{x+n}{n}} = \sum_{k=0}^n \frac{(-1)^k \binom{n}{k}}{x+k}$

The first of these equalities is straightforward by the definition of binomial coefficients. To prove the second, we make use of partial fractions. We write the expansion

$\displaystyle \prod_{k=0}^n \frac{1}{x+k} = \sum_{k=0}^n \frac{A_k}{x+k},$

where the coefficients $A_k$ are to be determined. Multiplying both sides by $(x+k')$ for some $k' \in \{0, 1, \ldots, n\}$,

\displaystyle \begin{aligned} (x+k') \prod_{k=0}^n \frac{1}{x+k} &= \sum_{k=0}^n \frac{A_k(x+k')}{x+k}\\\hbox{i.e.}\quad \prod_{\substack{k=0\\k \neq k'}}^n \frac{1}{x+k} &= A_{k'} + \sum_{\substack{k=0\\k \neq k'}}^n \frac{A_k(x+k')}{x+k}.\end{aligned}

Then setting $x = -k$ in both sides gives

\displaystyle \begin{aligned} \prod_{\substack{k=0\\k \neq k'}}^n \frac{1}{k-k'} &= A_{k'} + \sum_{\substack{k=0\\k \neq k'}}^n \frac{A_k(0)}{x+k}\\&= A_{k'}\\\hbox{i.e.}\quad A_{k'} &= \frac{1}{-k'} \cdot \frac{1}{-k'+1}\cdot \ldots \cdot \frac{1}{-1} \cdot \frac{1}{1}\cdot \frac{1}{2}\cdot \ldots \cdot \frac{1}{n-k'}\\&= \frac{(-1)^{k'}}{k'! (n-k')!}\\&= \frac{(-1)^{k'}}{n!} \binom{n}{k'},\end{aligned}

As a few examples of this identity we have:

\displaystyle \begin{aligned}n=1&:& \frac{1}{x(x+1)} &= \frac{1}{x} - \frac{1}{x+1}\\n = 2&:& \frac{1}{x(x+1)(x+2)} &= \frac{1}{2} \left[ \frac{1}{x} - \frac{2}{x+1} + \frac{1}{x+2} \right]\\n=3&:& \frac{1}{x(x+1)(x+2)(x+3)} &= \frac{1}{6} \left[ \frac{1}{x} - \frac{3}{x+1} + \frac{3}{x+2} - \frac{1}{x+3}\right] \end{aligned}

I had previously posted an identity with similar, but trickier derivation here.

## July 12, 2020

### Charting Test and ODI cricket performances over consecutive innings

Filed under: cricket,sport — ckrao @ 2:23 am

Adding to my earlier blog post about the highest proportion of test scores above m after n innings, I have created some new interactive charts for best streaks early and mid-career in both bowling and batting in tests and ODIs. The data is mostly from https://data.world/cclayford/cricinfo-statsguru-data.

#### 4. ODI bowling

Below are shown a few sample charts. Clicking on the chart will take you to a new page where you can interact further.

Test batting:

Test bowling:

ODI batting:

ODI bowling:

## July 8, 2020

### The largest parallelogram in a triangle

Filed under: mathematics — ckrao @ 11:59 pm

In this post we find the largest parallelogram, rhombus, rectangle and square that can be contained in a given triangle. We will see that in the first three cases we can achieve half the area of the triangle but no more, while it is generally less than this for a square.

#### 1. The largest parallelogram inside a triangle

It can be readily seen that one can obtain a parallelogram having half the area of a triangle by connecting a vertex with the three midpoints of the sides. (This has half the base length of the triangle and half its height.)

Is it possible to obtain a larger parallelogram? As outlined in [1], if two or fewer vertices of the parallelogram are on sides of the triangle, a smaller similar triangle can be created by drawing a line parallel to the triangle’s side through a vertex of the parallelogram that is interior triangle. (This is done three times in the figure below.) This reduces the problem to the next case.

We are left to consider the case where three or more vertices of the parallelogram on the triangle. We can draw a line from a vertex to the opposite side parallel to a pair of sides (in the figure below $AH$ is drawn parallel to $DG$), thus dissecting the triangle into two. Each of the smaller triangles then has an inscribed parallelogram where two of the vertices are on a side of each triangle. Then by drawing lines parallel to sides if required, we create two sub-problems each having four vertices of the parallelogram on the sides of the triangle.

Finally, if all four vertices of the parallelogram are on sides of the triangle, by the pigeonhole principle, two of them are on a side (say $BC$ as shown in the figure below). In this figure, if we let $AD/AB = k$ and the height of ABC from $BC$ be $h_a$, then by the similarity of triangles $ADE$ and $ABC$, $DE = k.BC$ and $\triangle ADE$ has height $kh_a$. Then the area of the parallelogram $DEFG$ is $DE \times (1-k)h_a = k(1-k) DE.h_a$ which is $2k(1-k)$ times the area of $\triangle ABC$. This quantity has maximum value 1/2 when $k=1/2$ so we conclude that the parallelogram does not exceed half the triangle’s area.

#### 2. The largest rhombus in a triangle

Constraining sides of the parallelogram to be equal (forming a rhombus), we claim that the largest rhombus that can be inscribed in a triangle is also half its area. This can be formed with two of the vertices on the second longest side of the triangle. Suppose $a \geq b \geq c$ are the sides of the triangle with $b=AC$ the second longest side length. Then let the segment $MN$ joining the midpoints of $AB$ and $BC$ form one side of the rhombus of length $b/2$. It remains to be shown that there exist parallel segments of this same length from $M, N$ to $AC$. The longest possible such segment has length $NC = a/2$ and the shortest has length $h_b/2$, half the length of the altitude of $\triangle ABC$ from $B$. We wish to show that $h_b/2 \leq b/2 \leq a/2$. This follows from

$\displaystyle h_b = c \sin A \leq c \leq b \leq a.$

The area of this rhombus is clearly half the area of the triangle as it has half the length of its base and half the height.

#### 3. The largest rectangle in a triangle

Here if $BC$ is the longest side of the triangle we form the rectangle from midpoints $D, E$ of $AB$ and $AC$ respectively, dropping perpendiculars onto $BC$ forming rectangle $DEFG$:

The area of this rectangle is half the area of the triangle as it has half the length of its base and half the height.

Interestingly the reflections of the vertices of the triangle in the sides of the rectangle coincide, showing a paper folding interpretation of this result [2].

Since rhombuses and rectangles are special cases of parallelograms and we found that inscribed parallelograms in a triangle occupy no more than half its area, the rhombus and rectangle constructions here are optimal.

#### 4. The largest square in a triangle

Here we shall see that the best we can do may not be half the area of the triangle. As before, if two or fewer vertices of the square are not on the sides of the triangle it is possible to scale up the square (or scale down the triangle) so that three of the square’s vertices are on the sides. We claim that the largest square must have two of its vertices on a side of the triangle. Suppose this is not the case and we have the figure below.

Consider squares $LMNO$ inscribed in $\triangle ABC$ so that one vertex $L$ is on $AB$, $M$ is on $BC$ and $O$ is on $CA$. We claim that the largest such square is either $DEFG$ (two vertices on $BC$) or $HIJK$ (two vertices on $AC$). Suppose on the contrary that neither of these squares is the largest. Then we make use of the fact that all 90-45-45 triangles $LMO$ inscribed in $\triangle ABC$ have a common pivot point $P$. This is the point at the intersection of the circumcircles of triangles $OAL$, $LBM$ and $MCO$. To show these circles intersect at a single point, we can prove that if the circumcircles of triangles $OAL$ and $LBM$ intersect at $P$ then the points $O,P,M,C$ are cyclic by the following equality:

\displaystyle \begin{aligned} \angle MPO &= 360^{\circ} - \angle LPM - \angle OPL\\ &= (180^{\circ} - \angle LPM) + (180^{\circ} - \angle OPL)\\ &= \angle B + \angle A \\ &= 180^{\circ} - \angle C,\end{aligned}

where the second last equality makes use of quadrilaterals $BMPL$, $ALPO$ being cyclic.

\displaystyle \begin{aligned}\angle BPC &= \angle BPM + \angle MPC\\ &= \angle BLM + \angle MOC\\ &= (\angle LAM + \angle AML) + (\angle MAO + \angle OMA)\\&=(\angle LAM + \angle MAO ) + (\angle AML + \angle OMA )\\&= \angle BAC + \angle OML\\&=\angle A + 45^{\circ}, \end{aligned}

with similar relations for $\angle APB$ and $\angle CPA$. Hence $P$ is the unique point satisfying

\displaystyle \begin{aligned}\angle APB &= \angle ACB + 90^{\circ}\\\angle BPC &= \angle BAC + 45^{\circ}\\\angle CPA &= \angle CBA +45^{\circ}.\end{aligned}

(Each equation defines a circular arc, they intersect at a single point. Note that $P$ may be outside triangle $ABC$.) This point is the centre of spiral similarity of 90-45-45 triangles $LMO$ with $L, M, O$ respectively on the sides $AB, BC, CA$ of the triangle. Consider the locus of the points of the square as $L, M, O$ vary on straight line segments pivoting about $P$. It follows that the fourth point of the square $N$ also traces a line segment, between the points $F$ and $J$ so as to be contained within the triangle.

As the side length of the square is proportional to the distance of a vertex to its pivot point, the largest square will be where $NP$ is maximised. We have seen that the point $N$ varies along a line segment, so $NP$ will be maximised at one of the extreme points – either when $N=F$ or $N=J$. We therefore conclude that the largest square inside a triangle will have two points on a side.

If the triangle is acute-angled, by calculating double the area of the triangle in two ways, the side length $x$ of a square on the side of length $a$ with altitude $h_a$ is derived as

\displaystyle \begin{aligned}ah_a &= 2x^2 + (a-x)x + x(h_a-x)\\&=2x^2 + ax - x^2 + xh_a - x^2\\&= ax + xh_a\\Rightarrow \quad x &= \frac{ah_a}{a + h_a}.\end{aligned}

If the triangle is obtuse-angled, the square erected on a side may not touch both of the other two sides. In the figure below the side length of square $BDEF$ is the same as if $A$ were moved to $G$, where $\triangle GBC$ is right-angled. In this case the square’s side length is $BC.BG/(BC + BG)$.

The largest square erected on a side may be constructed using the following beautiful construction [2]: simply erect a square CBDE external to the side $BC$ and find the intersection points $F = AD \cap BC, G = AE \cap BC$.

These points define the base of the square to be inscribed since by similar triangles

$\displaystyle \frac{IF}{BD} = \frac{AF}{AD} = \frac{FG}{DE} = h_a/(a + h_a)$

so that

$\displaystyle IF = FG = \frac{ah_a}{a + h_a}.$

One can use this interactive demo to view the largest square in any given triangle. One needs to find the largest of the three possibilities of the largest square erected on each side. In the acute-angled-triangle case, the largest square is on the side that minimises the sum of that side length and its corresponding perpendicular height – as their product is fixed as twice the triangle’s area, this will occur when the side and height have minimal difference. For a right-angled triangle with legs $a, b$ and hypotenuse $c$, we wish to compare the quantities $(a+b)$ and $(c+h)$, the two possible sums of the base and height of the triangle. We always have $a + b - c = 2(s-c) = 2r < h$ because the diameter of the incircle of the triangle is shorter than the altitude from the hypotenuse (i.e. the incircle is inside the triangle). We conclude that the largest square in a right-angled triangle is constructed on its two legs rather than its hypotenuse.

#### References

[1] I. Niven, Maxima and Minima without Calculus, The Mathematical Association of America, 1981.

[2] M. Gardner, Some Surprising Theorems About Rectangles in Triangles, Math Horizons, Vol. 5, No. 1 (September 1997), pp. 18-22.

[3] Jaime Rangel-Mondragon “Largest Square inside a Triangle” http://demonstrations.wolfram.com/LargestSquareInsideATriangle/ Wolfram Demonstrations Project Published: March 7 2011

## April 16, 2020

### Highest proportion of test scores above m after n innings

Filed under: cricket,sport — ckrao @ 6:02 am

I created an interactive workbook with Tableau to determine the test batsmen who have had the highest proportion of innings scoring at least m runs after having played n test innings. Below are some screenshots for particular choices of m runs. The data is available from [1]. As expected Bradman comes up on top in many scenarios but it is interesting to see other names that appear up there.

(Click on the above image to go to the Tableau page if you wish to change the parameters. You can also select the “Innings by innings” tab to look up a player’s list of innings.)

Below we some examples for different m (full-career stats among players who have played at least 20 innings).

m=1: A total of 22 players have had an entire career of 20+ innings getting off the mark each time, with RA Duff (Australia, 1902-1905) the only to play 40 innings (note that JW Burke played 44 innings without a duck, but made 0 not out in one innings).

Angelo Mathews (SL) has managed just 2 ducks in his 154 innings to date.

m=10: Hobbs, Hutton, Kanhai and Sobers stand out here, having played over 100 innings and reaching double figures at least 80% of the time (Hobbs over 86%). Labuschagne, Hetmyer, Handscomb and Head are recent players to feature highly here.

m=25: Bradman starts to distance himself from the rest. Hammond, Smith, Sobers and de Villiers also impress here.

m=50: Smith has matched Barrington’s career figures of 50+ starts. Sutcliffe had 33 50+ scores in his first 64 innings, the same as Bradman.

m=75: Barrington’s numbers are amazing here and Katich is ahead of Kohli, Tendulkar and Lara.

m=100: Smith and Kohli are currently higher than Sangakkara, the highest among recent retirees.

m=125: Only Bradman (6) had more 140+ scores than Labuschagne after playing 23 test innings, equal with Graeme Smith (who had 4 150+ scores in his first 17 innings!).

m=150: Bradman is so far ahead of the rest here. Lara and Sangakkara are just one behind Tendulkar with the most 150+ scores despite almost 100 fewer innings.

m=175: Again Lara and Sangakkara have the same number of 175+ scores (15).

m=200: Kohli is similar to Hammond’s career figures at this stage, with 6 of his 7 double centuries coming within 33 innings between July 2016 and December 2017.

## December 7, 2019

### Coordinates of special points of the 3-4-5 triangle

Filed under: mathematics — ckrao @ 3:40 am

One thing I observed is that the 3-4-5 triangle is rather attractive in solving problems using coordinates. If the vertices are placed at (0,3), (0,0) and (4,0) the following are the coordinates of points and equations of some lines of interest.

Line AC: $x/4 + y/3 = 1$

Incentre: $(1, 1)$

Centroid: $(4/3, 1)$

Circumcentre: $(2, 1.5)$

Orthocentre: $(0, 0)$

Nine-point centre: $(1, 3/4)$ (midpoint of the midpoints of AB and BC)

Angle bisectors: $y = x, y=-2x +3, y=4/3-x/3$

Ex-centres (intersection of internal and external bisectors): $(3,- 3), (6, 6), (-2, 2)$

Lines joining the excentres (in red above): $y=-x, y=x/2 +3, y = 3(x-4)$

Altitude to the hypotenuse: $y = 4x/3$

Euler line: $y=3x/4$

Foot of altitude to the hypotenuse: $(36/25, 48/25)$ (where $x/4 + y/3 = 1$ intersects $y=4x/3$)

Symmedian point (midpoint of the altitude to the hypotenuse [1]): $(18/25, 24/25)$

Contact points of incircle and triangle: $(1,0), (0,1), (8/5, 9/5)$

Gergonne point (intersection of Cevians that pass through the contact points of the incircle and triangle = the intersection of $y=3-3x$ and $y=1-x/4$): $(8/11, 9/11)$

Nagel point (intersection of Cevians that pass through the contact points of the ex-circles and triangle = the intersection of $y=3-x$ and $y=2-x/2$: $(2,1)$

#### Reference

[1] Weisstein, Eric W. “Symmedian Point.” From MathWorld–A Wolfram Web Resource. http://mathworld.wolfram.com/SymmedianPoint.html

## January 26, 2019

### 49+ °C temperatures in Australia

Filed under: climate and weather — ckrao @ 12:32 pm

Below is a list of recorded instances of maximum temperatures of 49 degrees Celsius or more in Australia, based on [1] and [2] from Australia’s Bureau of Meteorology. Out of the 48 52 occasions, 22 26 have occurred in this decade including 8 (so far) during this summer alone! I believe all the stations have been recording temperatures for at least 20 years except Port Augusta and Keith West (which both started in 2001). Edited: 20 Dec 2019

 Temperature (°C) Date Station Name State 50.7 2-Jan-60 Oodnadatta Airport SA 50.5 19-Feb-98 Mardie WA 50.3 3-Jan-60 Oodnadatta Airport SA 49.9 19-Dec-19 Nullarbor SA 49.8 19-Dec-19 Eucla WA 49.8 21-Feb-98 Emu Creek Station WA 49.8 13-Jan-79 Forrest Aero WA 49.8 3-Jan-79 Mundrabilla Station WA 49.7 10-Jan-39 Menindee Post Office NSW 49.6 12-Jan-13 Moomba Airport SA 49.5 19-Dec-19 Forrest WA 49.5 24-Jan-19 Port Augusta Aero SA 49.5 24-Dec-72 Birdsville Police Station QLD 49.4 21-Dec-11 Roebourne WA 49.4 16-Feb-98 Emu Creek Station WA 49.4 7-Jan-71 Madura Station WA 49.4 2-Jan-60 Marree Comparison SA 49.4 2-Jan-60 Whyalla (Norrie) SA 49.3 27-Dec-18 Marble Bar WA 49.3 2-Jan-14 Moomba Airport SA 49.3 9-Jan-39 Kyancutta SA 49.2 20-Dec-19 Keith West SA 49.2 24-Jan-19 Kyancutta SA 49.2 21-Feb-15 Roebourne Aero WA 49.2 10-Jan-14 Emu Creek Station WA 49.2 22-Dec-11 Onslow Airport WA 49.2 1-Jan-10 Onslow WA 49.2 11-Jan-08 Onslow WA 49.2 9-Feb-77 Mardie WA 49.2 1-Jan-60 Oodnadatta Airport SA 49.2 3-Jan-22 Marble Bar Comparison WA 49.2 11-Jan-05 Marble Bar Comparison WA 49.1 24-Jan-19 Tarcoola Aero SA 49.1 23-Jan-19 Red Rocks Point WA 49.1 13-Jan-19 Marble Bar WA 49.1 27-Dec-18 Onslow Airport WA 49.1 3-Jan-14 Walgett Airport AWS NSW 49.1 2-Jan-10 Emu Creek Station WA 49.1 18-Feb-98 Roebourne WA 49.1 23-Dec-72 Moomba SA 49 15-Jan-19 Tarcoola Aero SA 49 23-Jan-15 Marble Bar WA 49 13-Jan-13 Birdsville Airport QLD 49 9-Jan-13 Leonora WA 49 21-Dec-11 Roebourne Aero WA 49 1-Jan-10 Mardie WA 49 10-Jan-09 Emu Creek Station WA 49 11-Jan-08 Port Hedland Airport WA 49 11-Jan-08 Roebourne WA 49 12-Jan-88 Marla Police Station SA 49 6-Dec-81 Birdsville Police Station QLD 49 22-Dec-72 Marree SA

## December 30, 2018

### A collection of energy formulas

Filed under: science — ckrao @ 10:58 am

Energy is a quantity that is conserved as a consequence of the time translation invariance of the laws of physics. Below are some formulas calculating energy of different forms.

Kinetic energy is that associated with motion and is defined as $K = \frac{1}{2} mv^2 = \frac{p^2}{2m}$ for a particle with mass $m$, velocity $v$ and momentum $p$. If the mass is a fluid in motion (e.g. wind) with density $\rho$ and volume $A v t$ through cross-sectional area $A$, then $K = \frac{1}{2} At\rho v^3$.

Work is the result of a force $F$ applied over a displacement $\mathbf{s}$ and is given by the line integral

$\displaystyle W = \int_C \mathbf{F} . \mathrm{d}\mathbf{s} = \int_{t_1}^{t_2} \mathbf{F} . \frac{\mathrm{d}\mathbf{s}}{\mathrm{d}t} \ \mathrm{d}t= \int_{t_1}^{t_2} \mathbf{F}.\mathbf{v}\ \mathrm{d}t .$

This has the simple form $W = Fs \cos \theta$ when force is constant and displacement is linear where $\theta$ is the angle between the force and displacement vectors.

Using Newton’s 2nd law and the relation $\frac{\mathrm{d}}{\mathrm{d}t} (\mathbf{v}^2) = 2\mathbf{v}.\frac{\mathrm{d}\mathbf{v}}{\mathrm{d}t}$ this can be written as

$\displaystyle W = m\int_{t_1}^{t_2} \frac{d\mathbf{v}}{dt} . \mathbf{v} \mathrm{d}t = \frac{1}{2}m\int_{t_1}^{t_2} \frac{\mathrm{d}}{\mathrm{d}t} (\mathbf{v}^2) \mathrm{d}t = \frac{1}{2}m\int_{v_1^2}^{v_2^2} \mathrm{d}(\mathbf{v}^2) = \frac{1}{2}mv_2^2 - \frac{1}{2}mv_1^2.$

This is the work-energy theorem which says that work is the change in kinetic energy by a net force. It can also be written as $W = \int_{v_1}^{v_2} m \mathbf{v}.\mathrm{d}\mathbf{v} = \int_{p_1}^{p_2} \mathbf{v}.d\mathbf{p}$ where $\mathbf{p} = m\mathbf{v}$ is momentum.

The above has the rotational analogue $K = \frac{1}{2} I \omega^2$ where $I$ is moment of inertia and $\omega$ is angular velocity and the equation for work becomes

$\displaystyle W = \int_{t_1}^{t_2} \mathbf{T} . \mathbf{\omega}\ \mathrm{d}t$,

where $\mathbf{T}$ is a torque vector.

This has the simple form $W = Fr \omega = \tau \omega$ in the special case of a constant magnitude tangential force where $\tau = Fr$ is the torque resulting from force $F$ applied at distance $r$ from the centre of rotation.

Note that the time derivative of work is defined as power, so work can also be expressed as the time integral of power:

$W = \int P(t)\ \mathrm{d}t = \int_{t_1}^{t_2} \mathbf{F}.\mathbf{v}\ \mathrm{d}t.$

If the work done by a force field $\mathbf{F}$ depends only on a particle’s end points and not on its trajectory (i.e. conservative forces), one may define a potential function of position, known as potential energy $U$ satisfying $\mathbf{F} = -\nabla U$. By convention positive work is a reduction in potential, hence the minus sign. It then follows that in such force fields the sum of kinetic and potential energy is conserved.

Some types of potential energy:

• due to a gravitational field: $\mathbf{F} = -(GMm/r^2) \hat{r}, U = -GMm/r$, where $M, m$ are the masses of two bodies, $r$ the distance between their centre of masses and $G$ is Newton’s gravitation constant.
• due to earth’s gravity at the surface: $\mathbf{F} = -mg, U = mgh$ where $g \approx 9.8 ms^{-2}$ and $h$ is the object’s height above ground (small compared with the size of the earth).
• due to a spring obeying Hooke’s law: $\mathbf{F} = -kx, U = kx^2/2$ where $k$ is the spring constant and $x$ the displacement from an equilibrium position.
• due to an electrostatic field: $\mathbf{F} = q\mathbf{E} = (k_e qQ/r^2) \hat{r}, U = k_e qQ/r$ where $k_e$ is Coulomb’s constant $1/(4\pi \epsilon_0)$ and $q, Q$ are charges. This can be written as $U = qV$ where $V$ is a potential function measured in volts.
• for a system of point charges: $\displaystyle U =k_e \sum_{1 \leq i < j \leq n} \frac{q_i q_j}{r_{ij}}$.
• for a system of conductors: $U = \frac{1}{2} \sum_{i=1}^n Q_i V_i$ where the charge on conductor $i$ is $Q_i$ and its potential is $V_i$.
• for a charged dielectric: the above may be generalised to the volume integral $U = \frac{1}{2} \int_V \rho \Phi \ \mathrm{d}v$ where $\rho$ is charge density and $\Phi$ is the potential corresponding to the electric field.
• for an electric dipole in an electric field: $U = -\mathbf{p}.\mathbf{E}$ where $\mathbf{p}$ is directed from the negative to positive charge and has magnitude equal to the product of the positive charge and charge separation distance.
• for a current loop in a magnetic field: $U = -\mathbf{\mu}.\mathbf{B}$ where $\mathbf{\mu}$ is directed normal to the loop and has magnitude equal to the product of the current through the loop and its area.

In electric circuits the voltage drop across an inductance $L$ is $v = L di/dt$ and the current though a capacitance $C$ is $i = C dv/dt$. These inserted into the relationship $E = \int i(t)v(t) \ \mathrm{d}t$ lead to the formulas $E = \frac{1}{2}L(\Delta I)^2$ and $E = \frac{1}{2}C(\Delta V)^2$ for the energy stored in a capacitor and inductor respectively.

Also in electromagnetism the energy flux (flow per unit area per unit time) is the Poynting vector $\mathbf{S} = \mathbf{E} \times \mathbf{H}$, the cross product of the electric and magnetising field vectors. The electromagnetic energy in a volume $V$ is given by ([1])

$\displaystyle \frac{1}{2}\int_V \mathbf{B}.\mathbf{H} + \mathbf{E}.\mathbf{D} \ \mathrm{d}v$,

where $\mathbf{D}$ is the electric displacement field and $\mathbf{B}$ is the magnetic field. This is more commonly written as $\displaystyle \frac{1}{2} \int_V \epsilon_0 |E|^2 + |B^2|/\mu_0 \ \mathrm{d}v$ when the relationships $\mathbf{D} = \epsilon_0\mathbf{E}, \mathbf{B} = \mu_0 \mathbf{H}$ hold.

In special relativity energy is the time component of the momentum 4-vector. That is, energy and momentum are mixed in a similar way to how space and time are mixed at high velocities. Computing the norm of the momentum four-vector gives the energy-momentum relation

$E^2 = (pc)^2 + (m_0c^2)^2$.

This leads to $E = pc$ for massless particles (such as photons) and more generally $E = \gamma m_0 c^2$ , the mass-energy equivalence relation (here $\gamma = (1 - (v/c)^2)^{-1/2}$ and $m_0$ is rest mass).

In quantum mechanics the energy of a photon is also written as $E = hf = hc/\lambda$ (Planck-Einstein relation) where $h$ is Planck’s constant and $f, \lambda$ are frequency and wavelength respectively. Energies of quantum systems are based on the eigenstates of the Hamiltonian operator, an example of which is $\displaystyle {\hat {H}}=-{\frac {\hbar ^{2}}{2m}}\nabla^2+V(x)$.

Force is also equal to pressure times area, so another formula for work (e.g. done by an expanding gas) is the volume integral $W = \int p \mathrm{d}V$. In thermodynamics heat is energy transferred through the random motion of particles. The fundamental equation of thermodynamics quantifies the internal energy $U$ which disregards kinetic or potential energy of a system as a whole (only considering microscopic kinetic and potential energy):

$\displaystyle U = \int \left(T \text{d}S - p \mathrm{d}V + \sum_i \mu_i \mathrm{d}N_i \right)$

where $T$ is temperature, $S$ is entropy, $N_i$ is the number of particles and $\mu_i$ the chemical potential of species $i$. Similar formulas exist for other thermodynamic potentials such as Gibbs energy, enthalpy and Helmholtz energy.

The mean translational kinetic energy of a bulk substance is related to its temperature by $\bar{E} = \frac{3}{2}k_B T$ where $k_B$ is Boltzmann’s constant.

In thermal transfer the change in internal energy is given by $\Delta U = m C \Delta T$ where $m$ is mass and $C$ is the heat capacity which may apply to constant volume or constant pressure.

The power per unit area emitted by a body is given by the Stefan-Boltzmann law $P = A \epsilon \sigma T^4$ where $\epsilon$ is the emissivity (=1 for black body radiation) and $\sigma$ is the Stefan–Boltzmann constant. This equation may be used to determine the energy emitted by stars using their emission spectrum.

The latent heat (thermal energy change during a phase transition) of mass $m$ of a substance with specific latent heat constant $L$ is given by $Q = mL$.

Finally, the energy of a single wavelength of a mechanical wave is $\displaystyle \frac{1}{2} m\omega^2 A^2$ where $m$ the mass of a wavelength, $A$ the amplitude and $\omega$ the angular frequency [2]. This can be applied to finding the energy density of ocean waves for example [3].

#### References

[1] Poynting Vector. Brilliant.org. Retrieved 22:24, December 28, 2018, from https://brilliant.org/wiki/poynting-vector/

[2] Power of a Wave. Brilliant.org. Retrieved 21:23, December 30, 2018, from https://brilliant.org/wiki/power-of-a-wave/

[3] Wikipedia contributors, “Wave power,” Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Wave_power&oldid=875183814 (accessed December 30, 2018).

[4] Wikipedia contributors, “Work (physics),” Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Work_(physics)&oldid=874162163 (accessed December 30, 2018).

[5] Wikipedia contributors, “Potential energy,” Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Potential_energy&oldid=873393028 (accessed December 30, 2018).

[6] Wikipedia contributors, “Electric potential energy,” Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Electric_potential_energy&oldid=868852409 (accessed December 30, 2018).

[7] Wikipedia contributors, “Thermodynamic equations,” Wikipedia, The Free Encyclopedia, https://en.wikipedia.org/w/index.php?title=Thermodynamic_equations&oldid=865388931 (accessed December 30, 2018).

[8] H. Ohanian, Physics, 2nd edition, Norton & Company, 1989.

## June 12, 2018

### Rafael Nadal in best of five set matches on clay

Filed under: sport — ckrao @ 1:28 pm

Following Rafael Nadal‘s 11th French Open win, it’s worth looking at just how amazing his best-of-five set record on clay is. He now has a 111-2 win-loss record, with his only two losses against Söderling and Djokovic.

Win-loss breakdown by tournament (Masters tournament finals changed to best of 3 sets from 2007):

• French Open: 86-2
• Davis Cup: 18-0
• Barcelona Open: 2-0
• Monte Carlo Masters: 2-0
• Rome Masters: 2-0
• Stuttgart: 1-0

Win-loss breakdown by number of sets (overall he has won 331 and lost 36 completed sets so even winning a set against him is a big deal!):

• 5 sets: 4-0 (Coria, Federer, Isner, Djokovic – 5th set scores 7-6 (6), 7-6 (5), 6-4, 9-7 respectively)
• 4 sets: 22-1 (loss to Söderling)
• 3 sets: 83-1

Most common opponents (2 or more matches):

• Djokovic: 7-1 (lost 7 sets)
• Federer: 7-0 (lost 7 sets)
• Ferrer: 4-0 (lost 1 set)
• Almagro: 4-0
• Hewitt: 4-0 (lost 1 set)
• Söderling: 3-1 (lost 3 sets)
• Thiem: 3-0
• del Potro: 3-0 (lost 1 set)
• Gasquet: 3-0
• Seppi: 2-0 (lost 1 set)
• Murray: 2-0
• Roddick: 2-0 (lost 1 set)
• Coria: 2-0 (lost 3 sets)
• Ljubicic: 2-0
• Monaco: 2-0
• Bolelli: 2-0
• Wawrinka: 2-0 (same score of 6-2 6-3 6-1 both times)
• Bellucci: 2-0

Breakdown by set score (almost the same likelihood of winning a set 6-2, 6-3 or 6-4):

• 6-0: 26
• 6-1: 61
• 6-2: 68
• 6-3: 66
• 6-4: 67
• 7-5: 18
• 7-6: 24
• 9-7: 1
• 6-7: 9
• 5-7: 6
• 4-6: 8
• 3-6: 6
• 2-6: 3 (Federer in 2006 Rome, Söderling in 2009 French Open, Djokovic in 2012 French Open)
• 1-6: 3 (Federer in 2006 French Open, del Potro in 2011 Davis Cup, Djokovic in 2015 French Open)
• 0-6: 1 (Coria in 2005 Monte Carlo Masters)

(only one incomplete set 2-0 after which Pablo Carreno Busta retired)

## April 1, 2018

### A collection of binary grid counting problems

Filed under: mathematics — ckrao @ 3:52 am

The number of ways of colouring an m by n grid one of two colours without restriction is $2^{mn}$. The following examples show what happens when varying restrictions are placed on the colouring.

Example 1: The number of ways of colouring an m by n grid black or white so that there is an even number of 1s in each row and column is

$\displaystyle 2^{(m-1)(n-1)}.$

Proof: The first $m-1$ rows and $n-1$ columns may be coloured arbitarily. This then uniquely determines how the bottom row and rightmost column are coloured (restoring even parity). The bottom right square will be black if and only if the number of black squares in the remainder of the grid is odd, hence this is also uniquely determined by the first $m-1$ rows and $n-1$ columns. Details are also given here.

Example 2: The number of ways of colouring an m by n grid black or white so that every 2 by 2 square has an odd number (1 or 3) of black squares is

$\displaystyle 2^{m+n-1}.$

Proof: First colour the first row and first column arbitarily (there are $m+n-1$ such squares each with 2 possibilities). This uniquely determines how the rest of the grid must be coloured by considering the colouring of adjacent squares above and to the left.

By the same argument, the above is the same as the number of colouring an m by n grid black or white so that every 2 by 2 square has an even number (0, 2 or 4) of black squares.

Example 3: The number of ways of colouring an m by n grid black or white so that every 2 by 2 square has two of each type is

$\displaystyle 2^m + 2^n - 2.$

Proof: If there are two adjacent squares of the same colour with one above the other, the remaining squares of the corresponding two rows are uniquely determined as being the same alternating between black and white. The remainder of the grid is then determined by the colouring of first column ($2^m - 2$ possibilities where we omit the two cases of alternating colours down the first column). Such a grid cannot have two horizontally adjacent squares of the same colour. By a similar argument a colouring that has two adjacent colours with one left of the other can be done in $2^n-2$ ways. Finally we have the two additional configurations where there are no adjacent squares of the same colour, which is uniquely determined by the colour of the top left square. Hence in total we have $(2^m-2) + (2^n-2) + 2 = 2^m + 2^n - 2$ possible colourings.

This question for $m = n = 8$ was in the 2017 Australian Mathematics Competition and the general solution is also discussed here.

Example 4: The number of ways of colouring an m by n grid black or white so that each row and each column contain at least one black square is (OEIS A183109)

$\displaystyle \sum_{j=0}^m (-1)^j \binom{m}{j} (2^{m-j}-1)^n.$

Proof: First we count the number of colourings where a fixed subset of $j$ columns is entirely white and each row has at least one black square. The remaining $m-j$ columns and $n$ rows can be coloured in $(2^{m-j}-1)^n$ ways. To count colourings where each column has at least one black square we apply the principle of inclusion-exclusion and arrive at the above result.

Another inclusion-exclusion example shown here counts the number of 3 by 3 black/white grids in which there is no 2 by 2 black square. The answer is 417 with more terms for n by n grids in OEIS A139810.

Example 5: Suppose we wish to count the number of colourings of an m by n grid in which row i has $k_i$ black squares and column j has $l_j$ black squares ($i = 1, 2, \ldots m$, $j = 1, 2, \ldots, n$). Following [1], the number of ways this can be done is the coefficient of $x_1^{k_1}x_2^{k_2} \ldots x_m^{k_m}y_1^{l_1}y_2^{l_2}\ldots y_n^{l_n}$ in the polynomial

$\displaystyle \prod_{i=1}^m \prod_{j=1}^n (1 + x_i y_j).$

To see this note that expanding the product gives products of terms of the form $(x_i y_j)$ where such a term included corresponds to the $i$‘th row and $j$th column being coloured black. Hence the coefficient of $x_1^{k_1}x_2^{k_2} \ldots x_m^{k_m}y_1^{l_1}y_2^{l_2}\ldots y_n^{l_n}$ is the number of ways in which the system $\sum_{j=1}^n a_{ij} = k_i, \sum_{i=1}^m a_{ij} = l_j$ has a solution ($i = 1, 2, \ldots m$, $j = 1, 2, \ldots, n$) for $a_{ij}$ equal to 1 if and only if row $i$ and column $j$ are coloured black and 0 otherwise.

Let us evaluate this in the special case of 2 black squares in every row and every column for an n by n square grid (i.e. $k_i = l_j = 2$ and $m = n$). Picking two squares in each column to colour black means viewing the expansion as a polynomial in $y_1, \ldots, y_n$ the coefficient of $y_1^2y_2^2\ldots y_n^2$ has sums of products of $n$ terms of the form $x_ix_j$. Then using $[]$ notation to denote the coefficient of an expression, we have

\begin{aligned} \left[x_1^2x_2^2 \ldots x_n^2y_1^2y_2^2\ldots y_n^2 \right] \prod_{i=1}^n \prod_{j=1}^n (1 + x_i y_j) &= \left[x_1^2x_2^2 \ldots x_n^2 \right] \left( \sum_{i=1}^n\sum_{j=i+1}^n x_i x_j \right)^n\\&= \left[x_1^2x_2^2 \ldots x_n^2 \right] 2^{-n} \left( \left( \sum_{i=1}^n x_i\right)^2 - \sum_{i=1}^n x_i^2 \right)^n\\ &= \left[x_1^2x_2^2 \ldots x_n^2 \right] 2^{-n} \sum_{k=0}^n (-1)^k \binom{n}{k} \left( \sum_{i=1}^n x_i^2 \right)^k\left(\sum_{i=1}^n x_i\right)^{2(n-k)}\\ &= 2^{-n} \sum_{k=0}^n (-1)^k \binom{n}{k} \frac{n!}{(n-k)!} \frac{(2n-2k)!}{2^{n-k}}\\ &= 4^{-n} \sum_{k=0}^n (-1)^k \binom{n}{k}^2 2^k (2n-2k)!. \end{aligned}

Here the second last line follows from considering the number of ways that products of $k$ terms of the form $x_i^2$ arise in the product $\left( \sum_{i=1}^n x_i^2 \right)^k$ (which is $\frac{n!}{(n-k)!}$) and products of $(n-k)$ terms of the form $x_i^2$ can be formed in the product $\left(\sum_{i=1}^n x_i\right)^{2(n-k)}$ (which is $\frac{(2n-2k)!}{2^{n-k}}$).

For example, when $n=4$ this is equivalent to finding the coefficient of $a^2b^2c^2d^2$ in $(ab + bc + ac + bc + bd + cd)^4$. Products are either paired up in complementary ways such as in $ab.ab.cd.cd$ ($3 \times \binom{4}{2} = 18$ ways) or we have the three products $ab.bc.cd.ad, ab.bd.cd.ad, ac.bc.bd.ad$ ($3 \times 4! = 72$ ways). This gives us a total of 90 (this question appeared in the 1992 Australian Mathematics Competition). More terms of the sequence are found in OEIS A001499 and the 6 by 4 case (colouring two shaded squares in each row and three in each column in 1860 ways) appeared in the 2007 AIME I (see Solution 7 here).

Example 6: If we wish to count the number of grid configurations in which reflections or rotations are considered equivalent, we may make use of Burnside’s lemma that the number of orbits of a group is the average number of points fixed by an element of the group. For example, to find the number of configurations of 2 by 2 grids up to rotational symmetry, we consider the cyclic group $C_4$. For quarter turns there are $2^4$ configurations fixed (a quadrant determines the colouring of the remainder of the grid) while for half turns there are $2^8$ configurations as one half determines the colouring of the other half. This gives us an answer of

$\displaystyle \frac{2^{16} + 2.2^4 + 2^8}{4} = 16456,$

which is part of OEIS A047937. If reflections are also considered equivalent we need to consider the dihedral group $D_4$ and we arrive at the sequence in OEIS A054247.

If we want to count the number of 3 by 3 grids with four black squares up to equivalence, this is equivalent to the number of full noughts and crosses configurations. A nice video by James Grime explaining this is here (the answer is 23).

Example 7: The number of ways of colouring an m by n grid black or white so that the regions form 2 by 1 dominoes has the amazing form

$\displaystyle 2^{mn/2} \prod_{j=1}^{\lceil m/2 \rceil} \prod_{k=1}^{\lceil n/2 \rceil} \left(4 \cos^2 \frac{\pi j}{m+1} + 4 \cos^2 \frac{\pi k}{n+1}\right).$

For example, the 36 ways of tiling a 4 by 4 grid are given here. A proof of the above formula using the Pfaffian of the adjacency matrix of the corresponding grid graph is given in chapter 10 of [2].

#### References

[1] L. Comtet, Advanced Combinatorics: The Art of Finite and Infinite Expansions (pp 235-6), D. Reidel Publishing Company, 1974.

[2] M. Aigner, A Course in Enumeration, Springer, 2007.

## December 26, 2017

### The evolution of ODI team totals

Filed under: cricket,sport — ckrao @ 11:46 am

Over the years one day international cricket scores have been on the rise and this post intends to look into this in some detail. We shall restrict ourselves to first innings scores where the team batting first lasted exactly 50 overs. Hence games greater than 50 overs per team long or where a team was bowled out prematurely are omitted. There are 2349 (out of 3945) such matches according to this query on Cricinfo Statsguru and on average  7 wickets fall over the 50 overs. The plot below shows a scatter plot of the scores over time. The red curve shows that mean scores were steady around 225 during the 1980s and have been on the rise since 1990 so that now the mean score is approaching 300.

Note that the first data point in 1974 corresponds to a game that was reduced to 50 overs per side after originally intended to be a 55 over game.

If we slice the data into eras marked by calendar years of roughly equal numbers of games, the mean score had a slight slow-down in the rate of increase from 2008-2012, then accelerated again in the past five years.

 Era Number of matches Mean score batting first 1974-1994 427 229 1995-1999 383 247 2000-2003 368 257 2004-2007 380 267 2008-2012 393 272 2013-2017 398 288 1974-2017 2349 260

The histograms below show how rarely teams score less than 200 runs in recent times when using the full quota of 50 overs. In fact these days a team is more likely to score over 400 than below 200 if using the full quota of 50 overs!

Comparing the distribution of first innings winning versus losing scores we find that the mean scores are 275 vs 236 respectively with sample sizes 1392 vs 901 (34 games had no result and 22 were tied). Restricting to the past five years, the median score batting first for the full 50 overs in winning matches is exactly 300.

Interestingly if we break down the runs scatter plot by team, the trends are not the same across the board. In particular England and South Africa have had more dramatic increases in recent times than the other teams, especially compared with India, Pakistan, Sri Lanka and West Indies.

Restricting to the last five years (2013-2017), here are the mean first innings scores for each team based on the match result (assuming they bat the full 50 overs).

 Team Result mean score # matches Afghanistan lost 249 6 Afghanistan won 260 12 Australia lost 295 13 Australia n/r 253 3 Australia won 310 31 Bangladesh lost 263 16 Bangladesh won 275 15 Canada lost 230 3 England lost 282 13 England won 329 22 Hong Kong won 283 4 India lost 282 12 India won 310 27 Ireland lost 244 6 Ireland tied 268 1 Ireland won 289 3 Kenya lost 260 1 Netherlands lost 265 1 New Zealand lost 277 12 New Zealand tied 314 1 New Zealand won 308 27 P.N.G. lost 218 2 P.N.G. won 232 1 Pakistan lost 266 9 Pakistan n/r 296 1 Pakistan tied 229 1 Pakistan won 290 20 Scotland lost 238 6 Scotland won 284 8 South Africa lost 258 7 South Africa n/r 301 1 South Africa won 321 36 Sri Lanka lost 249 15 Sri Lanka n/r 268 2 Sri Lanka tied 286 1 Sri Lanka won 305 22 U.A.E. lost 279 3 U.A.E. won 267 3 West Indies lost 265 10 West Indies won 298 10 Zimbabwe lost 247 9 Zimbabwe tied 257 1 Zimbabwe won 276 1

The England and South Africa numbers stand out the most here in winning causes. Also Australia has a particularly high average score of 294 in losing causes. Sri Lanka has the largest difference (56 runs) between average winning and losing scores.

Edit: The following shows the mean scores in the 100 matches prior to and after key rule changes (still focusing on first innings 50-over scores). Note that in two of the three cases, the average scores reduced.

1. Restriction of 2 outside the 30-yard circle in the first 15 overs (’92 World Cup)
03 Jan 88 to 20 Jan 92: 231
12 Feb 92  to 16 Feb 94: 222
2. Introduction of Powerplay overs
13 Mar 04 to 30 Jun 05: 267
07 Jul 05 to 08 Sep 06: 267
3. Removal of powerplay, fifth fielder allowed outside the circle in the last ten overs
17Aug 14 to 24 Jun 15: 301
10 Jul 15 to 19 Jan 17: 289
Next Page »

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