Let $f(x)=\sum\_{n\geq 0}\frac{1}{n!n!}x^{n}$, is there an explicit formula for $f(x)$?



Is there an explicit formula for $\sum x^n/(n!)^2$?  
This is, for $x \ge 0$, a special case of the modified Bessel function of the first kind. Have a look here: <http://mathworld.wolfram.com/ModifiedBesselFunctionoftheFirstKind.html>
$$
\sum\_{n = 0}^{\infty} \frac{x^{n}}{(n!)^{2}} = \mathrm{I}\_0 \left(2 \sqrt{x}\right)
$$


