  In this paper we study $p$-adic dynamical systems generated by the function
$f(x)={a\over x^2}$ in the set of complex $p$-adic numbers. We find an explicit
formula for the $n$-fold composition of $f$ for any $n\geq 1$. Using this
formula we give fixed points, periodic points, basin of attraction and Siegel
disk of each fixed (periodic) point depending on parameters $p$ and $a$.
