Let us consider the sum of a function series $$ \sum _{n=1}^{\infty } \frac{z^n}{\left(z^n+1\right)^n}.$$
This series absolutely converges if Abs[z] < 1
or Abs[z] > 1
or z==1
in view of the results of
DiscreteMaxLimit[(Abs[z]^n/Abs[(1 + z^n)]^n)^(1/n), n -> Infinity,
Assumptions -> Abs[z] < 1]
Abs[z]
and
DiscreteMaxLimit[(Abs[z]^n/Abs[(1 + z^n)]^n)^(1/n), n -> Infinity,
Assumptions -> Abs[z] > 1]
0
and the root test. Its sum presents two different analytic functions having the unit disk $\{z:|z| < 1\}$ and its exterior $\{z:|z| > 1\}$ as the natural domains of existence (see Encyclopedia of Mathematics for info) since the term $\frac{z^n}{\left(z^n+1\right)^n}$ has $n$ poles of order $n$ uniformly distributed on the unit circumference.
ComplexPlot3D[Sum[z^n/(1 + z^n)^n, {n, 1, 20}], {z, 2}]
illustrates it.
In 14.0 on Windows 10 I try to plot this function for real values of $x$ from $0$ to $2$.
Here are my not very successful attempts taking a lot of time.
Plot[Sum[x^n/(1 + x^n)^n, {n, 1, Infinity}], {x, 0, 2}, PlotRange -> All]
Plot[NSum[x^n/(1 + x^n)^n, {n, 1, Infinity}], {x, 0, 2},
PlotRange -> All, WorkingPrecision -> 25]
Replacing the function under consideration by its partial sums, I obtain
Plot[{Sum[x^n/(1 + x^n)^n, {n, 1, 100}], Sum[x^n/(1 + x^n)^n, {n, 1, 1000}],
Sum[x^n/(1 + x^n)^n, {n, 1, 10000}]}, {x, 0, 2}, PlotRange -> All]
and
General::munfl: 0.0000408571^71 is too small to represent as a normalized machine number; precision may be lost.
The latest approach is not reliable (think of Sum[x^n,{n,1,Infinity}]
and Sum[x^n,{n,1,1000}]
). The hard piece is that
Plot[{Sum[x^n/(1 + x^n)^n, {n, 1, 100}], Sum[x^n/(1 + x^n)^n, {n, 1, 1000}],
Sum[x^n/(1 + x^n)^n, {n, 1, 10000}],
58 - 163 x + 156 x^2 - 50 x^3,
Sum[x^n/(1 + x^n)^n, {n, 1, 20000}]]}, {x, 0.95, 1.05}, PlotRange -> All]
General::munfl: 0.950002^13812 is too small to represent as a normalized machine number; precision may be lost.
Plot[NSum[x^n/(1 + x^n)^n, {n, 1, Infinity}], {x, 0.95, 1.05},
PlotRange -> All, WorkingPrecision -> 15, MaxRecursion -> 4, PerformanceGoal -> "Quality"]
NumericalMath`NSequenceLimit::seqlim: The general form of the sequence could not be determined, and the result may be incorrect.
We see the last two plots differ for $x<1$. 58 - 163 x + 156 x^2 - 50 x^3
is nothing but the wrong result of Series[Sum[x^k/(1 + x^k)^k, {k, 1, Infinity}], {x, 1, 3}] // Normal
: if this series were convergent , then there is an analytic continuation through 1
.
So the question is: how to reliably plot the function under consideration on the real range $[0,2]$?
NMaximize[{Sum[x^n/(1 + x^n)^n, {n, 51, Infinity}], x >= 0 && x <= 2}, x]
returns the input and a warning "NMaximize::nnum: The function value -\!(*UnderoverscriptBox[([Sum]), (n = 51), ([Infinity])](*SuperscriptBox[(0.09146359257542153\), \(n\)]\ \*SuperscriptBox[\((1 + \*SuperscriptBox[\(0.09146359257542153
), (n)])), (-n)])) is not a number at {x} = {0.0914636}.'. $\endgroup$