1. Number TheoryIn mathematics, a series is the sum of the terms of an infinite sequence of numbers. More precisely, an infinite sequence ( a 1 , a 2 , a 3 , … ) {\displaystyle (a_{1},a_{2},a_{3},\ldots )} defines a series S that is denoted S = a 1 + a 2 + a 3 + ⋯ = ∑ k = 1 ∞ a k .... from wikipedia.org