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I think

$f(x) = \sum_{n=1}^{\infty}(-1)^n \frac{x^{2n-1}}{(2n)! \ln 2n}$ is totally monotone.

Bernstein's theorem and his integral transform convinced me.

Am I correct ?

Probably related or useful is a function like

$f(x,k) = \sum_{n=1}^{\infty}(-1)^n \frac{x^{2n-1}}{(2n)! \ln (2n+k)}$

which follows or almost follows from differentiating.

This is a follow-up of the question

A curious limit for $-\frac{\pi}{2}$

about the same function.

mick
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    What is the definition of totally monotone? – A. P. Jan 21 '23 at 00:09
  • @A.P. See the wiki : https://en.wikipedia.org/wiki/Bernstein%27s_theorem_on_monotone_functions

    so basically for $ x> 0$ the derivatives alternate in sign.

    – mick Jan 21 '23 at 23:25
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    this is also called completely monotonic and the book by Widder on Laplace transform has a lot about them – Conrad Jan 22 '23 at 00:01

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