Sum[a[j],{j,1,3}]

Input interpretation

sum_(j=1)^3 a(j)
sum_(j=1)^3 a(j) = a(1) + a(2) + a(3)

Result

a(1) + a(2) + a(3) = a(1) + a(2) + a(3)
POWERED BY THE WOLFRAM LANGUAGE