well now i'm discovering a new impresive mathematics.
As it was said you can handle this problem in this way: f(n)=nf(n-1)=> f(n-1)=f(n)/n, lets see.
So to handle f(-1) we do f(0)=0f(-1),(this will always be cero, if f(-1)#oo).
As is defined f(0)=1
then f(-1)=1/0=+oo
this solution must verify de ...