Function: agm
Section: transcendental
C-Name: agm
Prototype: GGp
Help: agm(x,y): arithmetic-geometric mean of x and y.
Doc: arithmetic-geometric mean of $x$ and $y$. In the
 case of complex or negative numbers, the optimal AGM is returned
 (the largest in absolute value over all choices of the signs of the square
 roots).  $p$-adic or power series arguments are also allowed. Note that
 a $p$-adic agm exists only if $x/y$ is congruent to 1 modulo $p$ (modulo
 16 for $p=2$). $x$ and $y$ cannot both be vectors or matrices.
