Hi! Normally recursively defined functions work well, but if I use a sum inside, this happens:

> y:=n->sum(y(k),k=0..n-1);

                                   n - 1
                    ...

Hi!

> define( f , diff( f(x,y) , x ) = f1x(x,y) , diff( f(x,y) , y ) = f1y(x,y) );

produces

> diff( f(x,y) , x );
0

> diff( f(x,y) , y );
0

> diff( f(x,x) , x );
f1y(x,x)

So how can I do it correctly?