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Exercise 793Calculus › Single-variable calculus › Series › Convergence testsL1Added 2 Nov 2026

Two alternating series with logarithms

#alternating series test#monotonicity#conditional convergence

Statement

Study the convergence, absolute and conditional, of
∑n≥2(−1)nln⁡nand∑n≥1(−1)nln⁡nn.\sum_{n\ge2}\frac{(-1)^n}{\ln n}\qquad\text{and}\qquad\sum_{n\ge1}(-1)^n\frac{\ln n}n.

Hint

Check the three hypotheses of the alternating series test. For the second, study x↦ln⁡xxx\mapsto\frac{\ln x}x.

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Solution

First series. bn=1ln⁡nb_n=\frac1{\ln n} is positive, decreasing (as ln⁡n\ln n increases) and tends to 00: the series converges. Since ln⁡n≤n\ln n\le n we have 1ln⁡n≥1n\frac1{\ln n}\ge\frac1n, so ∑1ln⁡n\sum\frac1{\ln n} diverges: the convergence is conditional.

Second series. Let f(x)=ln⁡xxf(x)=\frac{\ln x}x. Then f′(x)=1−ln⁡xx2<0f'(x)=\frac{1-\ln x}{x^2}<0 for x>ex>e, so (ln⁡nn)n≥3\left(\frac{\ln n}n\right)_{n\ge3} is decreasing, and it tends to 00. The series converges. Since ln⁡nn≥1n\frac{\ln n}n\ge\frac1n for n≥3n\ge3, the series of absolute values diverges: conditional convergence.

Remark. The alternating series test requires *eventual* monotonicity only: the first terms (here n=1,2n=1,2) do not matter.