# Pascal's simplex

In mathematics, Pascal's simplex is a generalisation of Pascal's triangle into arbitrary number of dimensions, based on the multinomial theorem.

## Generic Pascal's m-simplex

Let m (m > 0) be a number of terms of a polynomial and n (n ≥ 0) be a power the polynomial is raised to.

Let $\wedge^m$ denote a Pascal's m-simplex. Each Pascal's m-simplex is a semi-infinite object, which consists of an infinite series of its components.

Let $\wedge^m_n$ denote its nth component, itself a finite (m − 1)-simplex with the edge length n, with a notational equivalent $\vartriangle^{m-1}_n$.

### nth component

$\wedge^m_n = \vartriangle^{m-1}_n$ consists of the coefficients of multinomial expansion of a polynomial with m terms raised to the power of n:

$|x|^n=\sum_{|k|=n}{\binom{n}{k}x^k};\ \ x\in\mathbb{R}^m,\ k\in\mathbb{N}^m_0,\ n\in\mathbb{N}_0,\ m\in\mathbb{N}$

where $\textstyle|x|=\sum_{i=1}^m{x_i},\ |k|=\sum_{i=1}^m{k_i},\ x^k=\prod_{i=1}^m{x_i^{k_i}}$.

### Example for $\wedge^4$

Pascal's 4-simplex (sequence A189225 in OEIS), sliced along the k4. All points of the same color belong to the same n-th component, from red (for n = 0) to blue (for n = 3).

## Specific Pascal's simplices

### Pascal's 1-simplex

$\wedge^1$ is not known by any special name.

#### nth component

$\wedge^1_n = \vartriangle^0_n$ (a point) is the coefficient of multinomial expansion of a polynomial with 1 term raised to the power of n:

$(x_1)^n = \sum_{k_1=n} {n \choose k_1} x_1^{k_1};\ \ k_1, n \in \mathbb{N}_0$
##### Arrangement of $\vartriangle^0_n$
$\textstyle {n \choose n}$

which equals 1 for all n.

### Pascal's 2-simplex

$\wedge^2$ is known as Pascal's triangle (sequence A007318 in OEIS).

#### nth component

$\wedge^2_n = \vartriangle^1_n$ (a line) consists of the coefficients of binomial expansion of a polynomial with 2 terms raised to the power of n:

$(x_1 + x_2)^n = \sum_{k_1+k_2=n} {n \choose k_1, k_2} x_1^{k_1} x_2^{k_2};\ \ k_1, k_2, n \in \mathbb{N}_0$
##### Arrangement of $\vartriangle^1_n$
$\textstyle {n \choose n, 0}, {n \choose n - 1, 1}, \cdots, {n \choose 1, n - 1}, {n \choose 0, n}$

### Pascal's 3-simplex

$\wedge^3$ is known as Pascal's tetrahedron (sequence A046816 in OEIS).

#### nth component

$\wedge^3_n = \vartriangle^2_n$ (a triangle) consists of the coefficients of trinomial expansion of a polynomial with 3 terms raised to the power of n:

$(x_1 + x_2 + x_3)^n = \sum_{k_1+k_2+k_3=n} {n \choose k_1, k_2, k_3} x_1^{k_1} x_2^{k_2} x_3^{k_3};\ \ k_1, k_2, k_3, n \in \mathbb{N}_0$
##### Arrangement of $\vartriangle^2_n$
\begin{align} \textstyle {n \choose n, 0, 0} &, \textstyle {n \choose n - 1, 1, 0}, \cdots\cdots, {n \choose 1, n - 1, 0}, {n \choose 0, n, 0}\\ \textstyle {n \choose n - 1, 0, 1} &, \textstyle {n \choose n - 2, 1, 1}, \cdots\cdots, {n \choose 0, n - 1, 1}\\ &\vdots\\ \textstyle {n \choose 1, 0, n - 1} &, \textstyle {n \choose 0, 1, n - 1}\\ \textstyle {n \choose 0, 0, n} \end{align}

## Properties

### Inheritance of components

$\wedge^m_n = \vartriangle^{m-1}_n$ is numerically equal to each (m − 1)-face (there is m + 1 of them) of $\vartriangle^m_n = \wedge^{m+1}_n$, or:

$\wedge^m_n = \vartriangle^{m-1}_n \subset\ \vartriangle^m_n = \wedge^{m+1}_n$

From this follows, that the whole $\wedge^m$ is (m + 1)-times included in $\wedge^{m+1}$, or:

$\wedge^m \subset \wedge^{m+1}$

#### Example

        $\wedge^1$         $\wedge^2$        $\wedge^3$         $\wedge^4$

$\wedge^m_0$     1          1          1          1

$\wedge^m_1$     1         1 1        1 1        1 1  1
1          1

$\wedge^m_2$     1        1 2 1      1 2 1      1 2 1  2 2  1
2 2        2 2    2
1          1

$\wedge^m_3$     1       1 3 3 1    1 3 3 1    1 3 3 1  3 6 3  3 3  1
3 6 3      3 6 3    6 6    3
3 3        3 3      3
1          1


For more terms in the above array refer to (sequence A191358 in OEIS)

### Equality of sub-faces

Conversely, $\wedge^{m+1}_n = \vartriangle^m_n$ is (m + 1)-times bounded by $\vartriangle^{m-1}_n = \wedge^m_n$, or:

$\wedge^{m+1}_n = \vartriangle^m_n \supset \vartriangle^{m-1}_n = \wedge^m_n$

From this follows, that for given n, all i-faces are numerically equal in nth components of all Pascal's (m > i)-simplices, or:

$\wedge^{i+1}_n = \vartriangle^i_n \subset \vartriangle^{m>i}_n = \wedge^{m>i+1}_n$

#### Example

The 3rd component (2-simplex) of Pascal's 3-simplex is bounded by 3 equal 1-faces (lines). Each 1-face (line) is bounded by 2 equal 0-faces (vertices):

2-simplex   1-faces of 2-simplex         0-faces of 1-face

1 3 3 1    1 . . .  . . . 1  1 3 3 1    1 . . .   . . . 1
3 6 3      3 . .    . . 3    . . .
3 3        3 .      . 3      . .
1          1        1        .


Also, for all m and all n:

$1 = \wedge^1_n = \vartriangle^0_n \subset \vartriangle^{m-1}_n = \wedge^m_n$

### Number of coefficients

For the nth component ((m − 1)-simplex) of Pascal's m-simplex, the number of the coefficients of multinomial expansion it consists of is given by:

${(n-1) + (m-1) \choose (m-1)} + {n + (m - 2) \choose (m - 2)} = {n + (m - 1) \choose (m - 1)},$

that is, either by a sum of the number of coefficients of an (n − 1)th component ((m − 1)-simplex) of Pascal's m-simplex with the number of coefficients of an nth component ((m − 2)-simplex) of Pascal's (m − 1)-simplex, or by a number of all possible partitions of an nth power among m exponents.

#### Example

Number of coefficients of nth component ((m − 1)-simplex) of Pascal's m-simplex
m-simplex nth component n = 0 n = 1 n = 2 n = 3 n = 4 n = 5
1-simplex 0-simplex 1 1 1 1 1 1
2-simplex 1-simplex 1 2 3 4 5 6
3-simplex 2-simplex 1 3 6 10 15 21
4-simplex 3-simplex 1 4 10 20 35 56
5-simplex 4-simplex 1 5 15 35 70 126
6-simplex 5-simplex 1 6 21 56 126 252

Interestingly, the terms of this table comprise a Pascal triangle in the format of a symmetric Pascal matrix.

### Symmetry

(An nth component ((m − 1)-simplex) of Pascal's m-simplex has the (m!)-fold spatial symmetry.)

### Geometry

(Orthogonal axes k_1 ... k_m in m-dimensional space, vertices of component at n on each axe, the tip at [0,...,0] for n=0.)

### Numeric construction

(Wrapped n-th power of a big number gives instantly the n-th component of a Pascal's simplex.)

$\left|b^{dp}\right|^n=\sum_{|k|=n}{\binom{n}{k}b^{dp\cdot k}};\ \ b,d\in\mathbb{N},\ n\in\mathbb{N}_0,\ k,p\in\mathbb{N}_0^m,\ p:\ p_1=0, p_i=(n+1)^{i-2}$

where $\textstyle b^{dp} = (b^{dp_1},\cdots,b^{dp_m})\in\mathbb{N}^m,\ p\cdot k={\sum_{i=1}^m{p_i k_i}}\in\mathbb{N}_0$.