Solution of an equation involving even integers If $x$ is any positive even integer $> 1$. I compute: 
$$ c = x + x! $$
Now assume instead $c$ (even integer) is given, and I want to get back the value $x$.
Is it possible to find a simple expression for $x$ ?
(or, if not, a convergent procedure to find it)

The $x$ is very small compared to $x!$, so basically you want to invert the Gamma function.  See e.g. this MathOverflow question
