When looking at a sequence and series, we need to distinguish between the ones we want to add, and the ones we do not want to add. The common symbol used for addition with a sequence or a series is the summation symbol, seen as the symbol Σ. When discussing a series, this symbol is used to mean the sum of numbers. For example, for the series called x_n:

Also note that for any given series Σx_n, we have to also remember to associate it with the sequence of partial sums (s_n = x₁ + x₂ + … + x_n).