# Fun With The Pascal Triangle

### Contents

#### Blaise Pascal

Blaise Pascal (1623-1662) was a 17th century French mathematician, physicist, inventor and theologian. His*Traité du triangle arithmétique*(

*Treatise on Arithmetical Triangle*) was published posthumously in 1665. But this was not the first publication about the triangle. Various versions appear in Indian, Chinese, Persian, Italian and other manuscripts centuries before Pascal.

#### Binomial Coefficients

The*binomial coefficient*usually denoted by ${n} \choose {k}$ is the number of ways of picking $k$ unordered outcomes from $n$ possibilities. These coefficients appear in the expansion of the binomial $(x+1)^n$. For example, when $n = 7$

```
syms x
n = 7;
x7 = expand((x+1)^n)
```

x7 = x^7 + 7*x^6 + 21*x^5 + 35*x^4 + 35*x^3 + 21*x^2 + 7*x + 1Formally, the binomial coefficients are given by $${{n} \choose {k}} = \frac {n!} {k! (n-k)!}$$ But premature floating point overflow of the factorials makes this an unsatisfactory basis for computation. A better way employs the recursion $$ {{n} \choose {k}} = {{n-1} \choose {k}} + {{n-1} \choose {k-1}}$$ This is used by the MATLAB function

`nchoosek(n,k)`.

#### Pascal Matrices

MATLAB offers two Pascal matrices. One is symmetric, positive definite and has the binomial coefficients on the antidiagonals.P = pascal(7)

P = 1 1 1 1 1 1 1 1 2 3 4 5 6 7 1 3 6 10 15 21 28 1 4 10 20 35 56 84 1 5 15 35 70 126 210 1 6 21 56 126 252 462 1 7 28 84 210 462 924The other is lower triangular, with the binomial coefficients in the rows. (We will see why the even numbered columns have minus signs in a moment.)

L = pascal(7,1)

L = 1 0 0 0 0 0 0 1 -1 0 0 0 0 0 1 -2 1 0 0 0 0 1 -3 3 -1 0 0 0 1 -4 6 -4 1 0 0 1 -5 10 -10 5 -1 0 1 -6 15 -20 15 -6 1The individual elements are

P(i,j) = P(j,i) = nchoosek(i+j-2,j-1)And (temporarily ignoring the minus signs) for

`i`$\ge$

`j`

L(i,j) = nchoosek(i-1,j-1)The first fun fact is that

`L`is the (lower) Cholesky factor of

`P`.

L = chol(P)'

L = 1 0 0 0 0 0 0 1 1 0 0 0 0 0 1 2 1 0 0 0 0 1 3 3 1 0 0 0 1 4 6 4 1 0 0 1 5 10 10 5 1 0 1 6 15 20 15 6 1So we can reconstruct

`P`from

`L`.

P = L*L'

P = 1 1 1 1 1 1 1 1 2 3 4 5 6 7 1 3 6 10 15 21 28 1 4 10 20 35 56 84 1 5 15 35 70 126 210 1 6 21 56 126 252 462 1 7 28 84 210 462 924

#### Pascal Triangle

The traditional Pascal triangle is obtained by rotating P clockwise 45 degrees, or by sliding the rows of L to the right in half increments. Each element of the resulting triangle is the sum of the two above it.triprint(L)

1 1 1 1 2 1 1 3 3 1 1 4 6 4 1 1 5 10 10 5 1 1 6 15 20 15 6 1

#### Square Root of Identity

When the even numbered columns of`L`are given minus signs the matrix becomes a square root of the identity.

L = pascal(n,1) L_squared = L^2

L = 1 0 0 0 0 0 0 1 -1 0 0 0 0 0 1 -2 1 0 0 0 0 1 -3 3 -1 0 0 0 1 -4 6 -4 1 0 0 1 -5 10 -10 5 -1 0 1 -6 15 -20 15 -6 1 L_squared = 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1Here is an exercise for you. What is

`sqrt(eye(n))`? Why isn't it

`L`?

#### Cube Root of Identity

When I first saw this, I was amazed. Rotate L counterclockwise. The result is a cube root of the identity.X = rot90(L,-1) X_cubed = X^3

X = 1 1 1 1 1 1 1 -6 -5 -4 -3 -2 -1 0 15 10 6 3 1 0 0 -20 -10 -4 -1 0 0 0 15 5 1 0 0 0 0 -6 -1 0 0 0 0 0 1 0 0 0 0 0 0 X_cubed = 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 1

#### Sierpinski

Which binomial coefficients are odd? It's a fledgling fractal.```
odd = @(x) mod(x,2)==1;
n = 56;
L = abs(pascal(n,1));
spy(odd(L))
title('odd(L)')
```

#### Fibonacci

The sums of the antidiagonals of`L`are the Fibonacci numbers.

n = 12; A = fliplr(abs(pascal(n,1))) for k = 1:n F(k) = sum(diag(A,n-k)); end F

A = 0 0 0 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 0 0 0 1 2 1 0 0 0 0 0 0 0 0 1 3 3 1 0 0 0 0 0 0 0 1 4 6 4 1 0 0 0 0 0 0 1 5 10 10 5 1 0 0 0 0 0 1 6 15 20 15 6 1 0 0 0 0 1 7 21 35 35 21 7 1 0 0 0 1 8 28 56 70 56 28 8 1 0 0 1 9 36 84 126 126 84 36 9 1 0 1 10 45 120 210 252 210 120 45 10 1 1 11 55 165 330 462 462 330 165 55 11 1 F = 1 1 2 3 5 8 13 21 34 55 89 144

#### pi

The elements in the third column of lower triangular Pascal matrix are the*triangle numbers*. The n-th triangle number is the number of bowling pins in the n-th row of an array of bowling pins. $$t_n = {{n+1} \choose {2}}$$

L = pascal(12,1); t = L(3:end,3)'

t = 1 3 6 10 15 21 28 36 45 55Here's an unusual series relating the triangle numbers to $\pi$. The signs go + + - - + + - - .

pi - 2 = 1 + 1/3 - 1/6 - 1/10 + 1/15 + 1/21 - 1/28 - 1/36 + 1/45 + 1/55 - ...

```
type pi_pascal
```

function pie = pi_pascal(n) tk = 1; s = 1; for k = 2:n tk = tk + k; if mod(k+1,4) > 1 s = s + 1/tk; else s = s - 1/tk; end end pie = 2 + s;Ten million terms gives $\pi$ to 14 decimal places.

```
format long
pie = pi_pascal(10000000)
err = pi - pie
```

pie = 3.141592653589817 err = -2.398081733190338e-14

#### Matrix Exponential

Finally, I love this one. The solution to the (potentially infinite) set of ordinary differential equations $\dot{x_1} = x_1$ $\dot{x_j} = x_j + (j-1) x_{j-1}$ is $x_j = e^t (t + 1)^{j-1}$ This means that the matrix exponential of the simple diagonal matrixD = diag(1:7,-1)

D = 0 0 0 0 0 0 0 0 1 0 0 0 0 0 0 0 0 2 0 0 0 0 0 0 0 0 3 0 0 0 0 0 0 0 0 4 0 0 0 0 0 0 0 0 5 0 0 0 0 0 0 0 0 6 0 0 0 0 0 0 0 0 7 0is

expm_D = round(expm(D))

expm_D = 1 0 0 0 0 0 0 0 1 1 0 0 0 0 0 0 1 2 1 0 0 0 0 0 1 3 3 1 0 0 0 0 1 4 6 4 1 0 0 0 1 5 10 10 5 1 0 0 1 6 15 20 15 6 1 0 1 7 21 35 35 21 7 1

#### Thanks

Thanks to Nick Higham for`pascal.m`,

`gallery.m`and section 28.4 of N. J. Higham,

*Accuracy and Stability of Numerical Algorithms*, Second edition, SIAM, 2002. http://epubs.siam.org/doi/book/10.1137/1.9780898718027 Published with MATLAB® R2018a

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