Here's a solution:
Obviously for y<0: f(y)=0.
For 0<=y<=1: f(y)=exp(y)=e^y.
For 1<y: f(y)= 1 + INTEGRAL{f(u) [where u runs from (y-1) to y]}, which upon differentiation, gives f'(y)=f(y)-f(y-1). Now we can partition the interval [1,infinity) as [1,2)U[2,3)U[3,4)U...U[n,n+1)U.... so that for...