### Sum of products of n-tuples

Posted:

**Thu Oct 01, 2015 9:53 am**Let's say I have an array of numbers {a,b,c,d,e} and want to calculate the sum of product of all triples abc+abd+...+cde. I can calculate it very fast without generating all triples using this formula

F(n,k)= v[k]*F(n-1, k-1)+ F(n-1, k) with two loops

Now I have this array of groups {{a1,a2,a3..an}, {b1,b2,b3 ...bn}, {c1,c2...} } and every element of a triple must be in different group. a1*b2*c1 is a valid triple but a1*a2*b1 is not allowed because a1, a2 are in the same group. It's possible to generalize the above algorithm to find the sum of valid triples (quadruples or n-tuples)?

F(n,k)= v[k]*F(n-1, k-1)+ F(n-1, k) with two loops

Code: Select all

```
int[] values={a, b, c, d, e...};
int k = 3;
int[,] F = new int[values.Count, k];
...
for(int i = 0; i < values.Count; i++)
for(int j = 0; j < k && j <= i; j++)
F[i, j] = values[i] * F[i - 1, j - 1] + F[i - 1, j];
```