 Euler also seems to suggest differentiating the latter series term by term.In the modern view, the series 1 − 2"x" + 3"x"2 − 4"x"3 + ... does not define a function at , so that value cannot simply be substituted into the resulting expression.Since the function is defined for all , one can still take the limit as "x" approaches 1, and this is the definition of the Abel sum