# Pascal Triangle

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• In mathematics, 'Pascal's triangle is a triangular array of the binomial coefficients in a triangle. … The rows of Pascal's triangle are conventionally enumerated starting with row n = 0 at the top. The entries in each row are numbered from the left beginning with k = 0 and are usually staggered relative to the numbers in the adjacent rows. A simple construction of the triangle proceeds in the following manner. On row 0, write only the number 1. Then, to construct the elements of following rows, add the number directly above and to the left with the number directly above and to the right to find the new value. If either the number to the right or left is not present, substitute a zero in its place. For example, the first number in the first row is 0 + 1 = 1, whereas the numbers 1 and 3 in the third row are added to produce the number 4 in the fourth row. This construction is related to the binomial coefficients by Pascal's rule, which states that if $\displaystyle{ (x+y)^n=\sum_{k=0}^n{n \choose k}x^{n-k}y^{k} }$ then $\displaystyle{ {n \choose k} = {n-1 \choose k-1} + {n-1 \choose k} }$ for any nonnegative integer n and any integer k between 0 and n.[1] Pascal's triangle has higher dimensional generalizations. The three-dimensional version is called Pascal's pyramid or Pascal's tetrahedron, while the general versions are called Pascal's simplices.
1. The binomial coefficient $\displaystyle{ \scriptstyle {n \choose k} }$ is conventionally set to zero if k is either less than zero or greater than n.