How to draw the graph of f(x,y) = x * y ?

Dear All,

The function curve() draws the graph of functions from R to R. Is
there some homologous function to curve() to draw functions from R^2
to R?

No, you would have to write it yourself.  The basic idea is to use 
outer() to construct a matrix of function evaluations, then plot the 
matrix using persp, contour, image, rgl::persp3d, etc.  There are 
examples on the ?contour man page.

Duncan Murdoch