f:N-->N

f(0) = 1

$\displaystyle 1+\sum_{j=0}^{n-1} jf(j)$, n>=1

i.e.

1 + ((0*f*0) + (1*f*1) + (2 * f *2) ....(n-1)*f*(n-1))<-- I'm sure this isn't right

then f(4) =

1+ ((0*4*0) + (1*4*1) + (2 * 4 *2) + (3 * 4 *3))

= 1 + 0 + 4 + 16 + 36

= 55

Have I worked this correctly?

Thanks for your time.