# Wanted: simple invertible function with specified derivative properties

I'm looking for a positive function $F(x)$, defined for positive real numbers, with the following properties.

1. $F(x)$ is expressible with the standard computer math library routines;
2. $F(x)$ is invertible and its inversion expressible with the standard computer math library routines;
3. $F'(x)>0$ and $F''\le0$.
4. at $x\to\infty$: $F'\propto x^{-\gamma}$ with $1<\gamma<2$ a parameter;
5. at $x\to0$: $F'\to\mathrm{const}$ and $F''\to0$ (ideally $F''\propto x$ in this limit).

I couldn't find anything (despite some extensive search). The first condition really makes it hard. Of course, I could use numerical inversion, but this will be the second choice.

Why do I need this? I want to sample $N$ points at positive $x$ with number density $n\propto x^{-\gamma}$ at large $x$ and continuously differentiable $n(r)=F'(|r|)$ (in particular at $r=0$). I don't want to sample these positions randomly, but equidistantly in the cumulated number $\propto F(x)$, hence the requirement to invert $F(x)$. Also, I don't want to suffer from round-off error more than absolutely necessary (hence preferentially no numerical inversion).

edit So what did I try? $F(x)=x(1+x^{\gamma-1})^{1/(1-\gamma)}$ meets criteria 1-4, but $F''(x)\to\infty$ at $x\to0$

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I assume you also wnat $F$ itself to be "expressible with the standard computer math library routines"? – Hagen von Eitzen Feb 3 '14 at 18:34
Try $F(x) = 1-(1+x)^{1-\gamma}$. – copper.hat Feb 3 '14 at 18:58
@copper.hat Sorry, that fails: $F''(0)\neq0$. – Walter Feb 4 '14 at 8:27
@Walter: Oops, misread the question. – copper.hat Feb 4 '14 at 8:32