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Exercise 2034Calculus › Single-variable calculus › Series › Convergence testsL1Added 6 Nov 2026

Four series involving logarithms

#root test#comparison#equivalent#logarithm

Statement

Study the nature of the following series:

  1. ∑n≥21(ln⁡n)n\displaystyle\sum_{n\ge2}\frac{1}{(\ln n)^n};
  2. ∑n≥2Hnln⁡(n!)\displaystyle\sum_{n\ge2}\frac{H_n}{\ln(n!)} where Hn=1+12+⋯+1nH_n=1+\frac12+\cdots+\frac1n;
  3. ∑n≥11nln⁡(n)ln⁡ ⁣(1+1n)\displaystyle\sum_{n\ge1}\frac1n\ln(n)\ln\!\left(1+\frac1n\right);
  4. ∑n≥21(ln⁡n)ln⁡n\displaystyle\sum_{n\ge2}\frac{1}{(\ln n)^{\ln n}}.
Hint
  1. Cauchy's root test. 2. Compare ln⁡(n!)\ln(n!) with nln⁡nn\ln n and HnH_n with ln⁡n\ln n. 3. Use ln⁡(1+1/n)∼1/n\ln(1+1/n)\sim 1/n, then compare ln⁡n/n2\ln n/n^2 with 1/n3/21/n^{3/2}. 4. Write (ln⁡n)ln⁡n=nln⁡ln⁡n(\ln n)^{\ln n}=n^{\ln\ln n} and compare with 1/n21/n^2.
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Solution

(1) ∑1(ln⁡n)n\sum\frac{1}{(\ln n)^n}. By Cauchy's root test:
1(ln⁡n)nn=1ln⁡n⟶0<1.\sqrt[n]{\frac{1}{(\ln n)^n}}=\frac{1}{\ln n}\longrightarrow0<1.
The series converges.

(2) ∑Hnln⁡(n!)\sum\frac{H_n}{\ln(n!)}. First compare ln⁡(n!)\ln(n!) with nln⁡nn\ln n: for 1≤k≤n1\le k\le n, ln⁡k≤ln⁡n\ln k\le\ln n, hence
ln⁡(n!)=∑k=1nln⁡k≤nln⁡n.\ln(n!)=\sum_{k=1}^{n}\ln k\le n\ln n.
Therefore
Hnln⁡(n!)≥Hnnln⁡n.\frac{H_n}{\ln(n!)}\ge\frac{H_n}{n\ln n}.
Moreover Hn≥ln⁡nH_n\ge\ln n (comparison of the harmonic sum with ∫1ndxx\int_1^n\frac{dx}{x}), so
Hnnln⁡n≥ln⁡nnln⁡n=1n.\frac{H_n}{n\ln n}\ge\frac{\ln n}{n\ln n}=\frac1n.
Since ∑1n\sum\frac1n diverges, by comparison the series diverges.

(3) ∑1nln⁡(n)ln⁡(1+1/n)\sum\frac1n\ln(n)\ln(1+1/n). Since ln⁡(1+1/n)∼1n\ln(1+1/n)\sim\frac1n as n→∞n\to\infty, the general term satisfies
un=ln⁡nn⋅ln⁡ ⁣(1+1n)∼ln⁡nn2.u_n=\frac{\ln n}{n}\cdot\ln\!\left(1+\frac1n\right)\sim\frac{\ln n}{n^2}.
Now ln⁡nn2=o ⁣(1n3/2)\frac{\ln n}{n^2}=o\!\left(\frac{1}{n^{3/2}}\right) (since ln⁡n/n→0\ln n/\sqrt n\to0), and ∑n−3/2\sum n^{-3/2} converges (Riemann, α=3/2>1\alpha=3/2>1). By comparison, the series converges.

(4) ∑1(ln⁡n)ln⁡n\sum\frac{1}{(\ln n)^{\ln n}}. Rewrite
1(ln⁡n)ln⁡n=1exp⁡(ln⁡n⋅ln⁡ln⁡n)=1nln⁡ln⁡n.\frac{1}{(\ln n)^{\ln n}}=\frac{1}{\exp(\ln n\cdot\ln\ln n)}=\frac{1}{n^{\ln\ln n}}.
Since x↦ln⁡ln⁡xx\mapsto\ln\ln x is increasing and tends to +∞+\infty on [e,+∞[[e,+\infty[, there exists NN with ln⁡ln⁡n≥2\ln\ln n\ge2 for all n≥Nn\ge N. For such nn,
un≤1n2,u_n\le\frac{1}{n^2},
the general term of a convergent Riemann series. By comparison, the series converges.