Series
a = {1,2,3,4,5,6}
top=5

What I need
sum = 1*6 + 2*5 + 3*4

---

Definition of discrete convolution
(f*g)[n] = SUM[k=-inf..+inf] f(k)*g(k-n)

So,

(a*a)[top] = a(0)*(5) + a(1)*a(4) + a(2)*a(3) + a(3)*a(2) + a(4)*a(1) + a(5)*a(0)
= 1*6 + 2*5 + 3*4 + 4*3 + 5*2 + 6*1

Dividing by :2 gives the desired result

(a*a)[top] / 2 = 1*6 + 2*5 + 3*4