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:
n≥2∑(lnn)n1;
n≥2∑ln(n!)Hn where Hn=1+21+⋯+n1;
n≥1∑n1ln(n)ln(1+n1);
n≥2∑(lnn)lnn1.
Hint
Cauchy's root test. 2. Compare ln(n!) with nlnn and Hn with lnn. 3. Use ln(1+1/n)∼1/n, then compare lnn/n2 with 1/n3/2. 4. Write (lnn)lnn=nlnlnn and compare with 1/n2.
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Solution
(1) ∑(lnn)n1. By Cauchy's root test: n(lnn)n1=lnn1⟶0<1. The series converges.
(2) ∑ln(n!)Hn. First compare ln(n!) with nlnn: for 1≤k≤n, lnk≤lnn, hence ln(n!)=k=1∑nlnk≤nlnn. Therefore ln(n!)Hn≥nlnnHn. Moreover Hn≥lnn (comparison of the harmonic sum with ∫1nxdx), so nlnnHn≥nlnnlnn=n1. Since ∑n1 diverges, by comparison the series diverges.
(3) ∑n1ln(n)ln(1+1/n). Since ln(1+1/n)∼n1 as n→∞, the general term satisfies un=nlnn⋅ln(1+n1)∼n2lnn. Now n2lnn=o(n3/21) (since lnn/n→0), and ∑n−3/2 converges (Riemann, α=3/2>1). By comparison, the series converges.
(4) ∑(lnn)lnn1. Rewrite (lnn)lnn1=exp(lnn⋅lnlnn)1=nlnlnn1. Since x↦lnlnx is increasing and tends to +∞ on [e,+∞[, there exists N with lnlnn≥2 for all n≥N. For such n, un≤n21, the general term of a convergent Riemann series. By comparison, the series converges.