 Cesàro summation is one of the few methods that do not sum , so the series is an example where a slightly stronger method, such as Abel summation, is required.The series 1 − 2 + 3 − 4 + ... is closely related to Grandi's series .Euler treated these two as special cases of for arbitrary "n", a line of research extending his work on the Basel problem and leading towards the functional equations of what are now known as the Dirichlet eta function and the Riemann zeta function.