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Exercise 1470Calculus › Single-variable calculus › Series › Convergence testsL1Added 2 Nov 2026

The series of 1/(ln n)^(ln n)

#comparison test#exponential form#logarithm

Statement

Study ∑n≥21(ln⁡n)ln⁡n\displaystyle\sum_{n\ge2}\frac1{(\ln n)^{\ln n}}.

Hint

Write (ln⁡n)ln⁡n=eln⁡n⋅ln⁡ln⁡n=nln⁡ln⁡n(\ln n)^{\ln n}=e^{\ln n\cdot\ln\ln n}=n^{\ln\ln n}.

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Solution

Rewriting. 1(ln⁡n)ln⁡n=1nln⁡ln⁡n\dfrac1{(\ln n)^{\ln n}}=\dfrac1{n^{\ln\ln n}}.

Comparison. Since ln⁡ln⁡n→+∞\ln\ln n\to+\infty, there is n0n_0 (namely n0=⌈ee2⌉n_0=\lceil e^{e^2}\rceil) such that ln⁡ln⁡n≥2\ln\ln n\ge2 for n≥n0n\ge n_0. For such nn,
1nln⁡ln⁡n≤1n2.\frac1{n^{\ln\ln n}}\le\frac1{n^2}.
Hence the series converges.

Remark. The exponent is not constant but eventually exceeds any fixed number, so the series is much "more summable" than any pp-series. But note the threshold n0≈1618n_0\approx1618 already: the first terms are large, since ln⁡n\ln n is small.