V does not represent your function, it's just a parameter. As it is now you
are telling povray to do:
#declare Myfunction (0.1) (0.1*2)
sphere {x, 0.1)
which isn't correct.
try something like this:
#declare Myfunction = function (V) {V*2}
sphere {x, Myfunction (0.1)}
--
Jonathan.
