# Closed expression for alternating harmonically wrapped harmonic series $\sum_{n=1}^\infty (-1)^{n+1} H_{\frac{1}{n}}$

It is well known that the alternating harmonic sum $$\sum_{n=1}^\infty (-1)^{n+1} \frac{1}{n}$$ converges to $$\log(2)$$.

Now let us wrap $$\frac{1}{n}$$ with the harmonic number $$H_k$$ (continued analytically to real values $$k \to z$$ as e.g. in https://math.stackexchange.com/a/3058569/198592) and ask if the sum

$$s = \sum_{n=1}^\infty (-1)^{n+1} H_{\frac{1}{n}}$$

is convergent.

Since for $$n=1,2,3,...$$ we have $$H_{\frac{1}{n+1}}\lt H_{\frac{1}{n}}$$ and $$H_0=0$$ the sequence $$a_n = H_{\frac{1}{n}}$$ is monotonous and goes to zero. Hence from the Leibniz criterion convergence is guaranteed.

The numerical value is

$$N(s) \simeq 0.638288$$

I wonder if there is a closed form expression for $$s$$.

## 2 Answers

$$H_{1/n} = \frac{\zeta(2)}{n}-\frac{\zeta(3)}{n^2}+\frac{\zeta(4)}{n^3}-\frac{\zeta(5)}{n^4}+\ldots$$ hence $$\sum_{n\geq 1}\frac{(-1)^{n+1}}{n}H_{1/n} = \sum_{k\geq 2}(-1)^k \zeta(k)\eta(k)$$ where the series appearing in the RHS has to be intended à-la-Cesàro. Since $$\sum_{k\geq 2}(-1)^k (\zeta(k)-1)=\frac{1}{2},\qquad \sum_{k\geq 2}(-1)^k (\eta(k)-1)=2\log 2-\frac{3}{2}$$ we have $$\sum_{n\geq 1}\frac{(-1)^{n+1}}{n}H_{1/n} = 2\log 2-\frac{1}{2}+\sum_{k\geq 2}(-1)^k(\zeta(k)-1)(\eta(k)-1)$$ in the classical sense. By exploiting the generating functions for $$\zeta(k)-1$$ and $$\eta(k)-1$$ the last series can be written as an inner product, $$\frac{1}{2\pi}\int_{0}^{2\pi}f(e^{i\theta})\,d\theta$$, and numerically approximated in a efficient way. We also have that $$(-1)^{d+1}$$ is a multiplicative function, hence

$$\eta(k)\zeta(k)=\sum_{n\geq 1}\frac{1}{n^k}\sum_{d\mid n}(-1)^{d+1}=2\sum_{n\geq 1}\frac{d(n)}{n^k}-4\sum_{n\geq 1}\frac{d(n)}{(2n)^k}$$ and

$$\sum_{n\geq 1}\frac{(-1)^{n+1}}{n}H_{1/n} = \sum_{n\geq 1}\frac{d(n)}{(n+1)(2n+1)}$$ can be written as $$\sum_{n\geq 1}[x^{2n}]\left[\sum_{m\geq 1}\frac{x^{2m}}{1-x^{2m}}\right]\cdot 2\int_{0}^{1}x^{2n}(1-x)\,dx = 2\int_{0}^{1}(1-x)\sum_{m\geq 1}\frac{x^{2m}}{1-x^{2m}}\,dx$$ involving a classical Lambert series. The last integral function is given by the sum between $$\log(1-x)$$ and a continuous, bounded function on $$[0,1]$$, so the numerical approximation of the last integral is not difficult. The same techniques applies to $$\sum_{n\geq 1}(-1)^{n+1} H_{1/n} =\zeta(2)\log(2)+\sum_{k\geq 2}(-1)^{k+1}\zeta(k+1)\eta(k)$$ since $$\zeta(k+1)\eta(k)=(2-2^{1-k})\sum_{n\geq 1}\frac{\sigma(n)}{n^{1+k}},$$ $$\sum_{k\geq 2}(-1)^{k+1}\zeta(k+1)\eta(k)=-\sum_{n\geq 1}\frac{(3n+1)\sigma(n)}{n^2(1+n)(1+2n)}$$ and $$\sum_{n\geq 1}\sigma(n) x^n = \sum_{m\geq 1}\frac{x^m}{(1-x^m)^2}.$$ For the pointwise evaluation of the involved Lambert series I suggest the approach through $$\mathscr{M},\mathscr{M}^{-1}$$ outlined here by Marko Riedel. It turns out that the approximated evaluation of $$\sum_{n\geq 1}\frac{(-1)^{n+1}}{n}H_{1/n}$$ is related to the average order of $$d(n)$$ (tackled by Dirichlet's hyperbola method) and the approximated evaluation of $$\sum_{n\geq 1}(-1)^{n+1}H_{1/n}$$ is related to the average order of $$\sigma(n)$$. The magnitude of the error terms can be controlled through the Voronoi summation formula (about this, I can finally link the pages 126-144 of my notes); the optimal bounds are doomed to the depend on the shape of the zero-free region for the Riemann $$\zeta$$ function.

1. Jack D'Aurizio has given a contribution with many interesting relations to deep lying neighbouring fields.

He started from the expansion of the harmonic number $$H_z$$ around $$z=0$$.

This leads to a divergent sum

$$s_{JA} = \sum_{k=1}^\infty (-1)^{k+1} \eta(k)\zeta(k),\;\;\eta(k) = \left(1-2^{1-k}\right) \zeta (k)$$

whose partial sums oscillate between two finte values of order unity.

1. Here I propose another approach which avoids divergent series. It starts from the formula

$$H_{z} = \sum_{k=1}^\infty \frac{z}{k(k+z)}$$

leading after swapping the order of summation to

$$s_{WH} = \sum_{k=1}^\infty \frac{\Phi \left(-1,1,1+\frac{1}{k}\right)}{k^2}$$

where

$$\Phi(z,s,a) = \sum_{k=0}^\infty z^k(k+a)^{-s}$$

is the Lerch transcendent (http://mathworld.wolfram.com/LerchTranscendent.html).

The summands of $$s_{WH}$$ are positive and decreasing with $$k$$.

Since $$\lim_{k\to\infty}\Phi \left(-1,1,1+\frac{1}{k}\right) = \Phi \left(-1,1,1\right)=\log(2)$$ the sum is convergent and less than $$\zeta(2) \log(2) \simeq 1.14018$$.

1. We could also start from Euler's integral formula

$$H_{z} = \int_0^1 \frac{1-x^{z}}{1-x}\,dx$$

and replace the integrand letting $$z \to \frac{1}{n}$$

$$\frac{1-x^{1/n}}{1-x}$$

with its expansion about $$x=1$$ up to a given order. This then leads to a finite series of terms to be subtracted from $$\log(2)$$.

For example 10 terms give $$s_{order 10} = 0.639872$$ instead of the exact value $$N(s) \simeq 0.638288$$