Fibonacci series : 1,1,2,3,5,8,13..........
Its generating function is :
1*z0 + 1*z1 +2*z2 +3*z3 +5*z4+-----------∞ = G(z)
Above series can be rewritten as :
1 + (z +z2) + (z+z2)2 +....................∞ = G(z)
1/(1 - (z+z2) = G(z)
Hence generating function G(z) = 1/(1- z -z2)