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Sum of alternating-like series as definite integrals
Published in International Journal of Mathematical Education in Science and Technology, 2022
Sergio A. Carrillo
The goal of the present note is to contribute to this point by summing the family of series
where the sequence is k-periodic, i.e. , for all integers and satisfies
We usually use two well-known methods. The first method, to establish the convergence of (1) is Dirichlet's test (Rudin, 1976, p. 70): if is a decreasing sequence of real numbers such that as , and a sequence of complex numbers with bounded partial sums, then converges. The case corresponds to Leibniz's test (also called alternating series test). Our second tool is Abel's continuity theorem for power series (Rudin, 1976, p. 174): if the series of complex numbers converges, then the power series converges for and
We will prove that this limit reduces the sum of (1) to the definite integral
which is finite since the factor 1−t can be eliminated from the numerator and denominator of fraction, thanks to the condition (2).