Capital ∑ is used for mathematical summation symbols, such as ∑Pi, where i= 1, 2, ..., t, which is the sum ...+angle of P 1+P2+.

The sum symbol ∑ is the capital of the Greek letter Sigma, which means that all the numbers on its right, here, 1/n2, are added together. The "n= 1" below ∑ means that we start with n= 1, and according to the convention, n is a positive integer that increases in turn. The ∞ above ∑ means "infinity", telling us to add these numbers constantly until forever.