It can be done elegantly with tail recursion.
series[a_, b_, n_] := Flatten[helper[a, b, n, {}]] helper[_, _, 0, s_] := s helper[a_, b_, n_, s_] := helper[b, a + b, n - 1, {s, a}] series[0, 2, 10]
{0, 2, 2, 4, 6, 10, 16, 26, 42, 68}
When the 1st two arguments are set to 1, series gives the Fibonacci series.
series[1, 1, 10] == Table[Fibonacci[i], {i, 10}]
True