How do I write this in mathematical symbols: "Result is equal to the sum of product of $a(i)$ and $b(j)$ when $i,j$ are natural numbers and satisfy the property $i+j = k$"?
I forgot to add one more constraint that :
i and j are in the range [0,n] k is in the range [0,2n]
n is a natural number.
Sorry for the late update.