Number of words of length n in a simple grammar.

Terms

a(0) =1a(1) =1a(2) =3a(3) =8a(4) =23a(5) =68a(6) =207a(7) =644a(8) =2040a(9) =6558a(10) =21343a(11) =70186a(12) =232864a(13) =778550a(14) =2620459a(15) =8872074a(16) =30195288a(17) =103246502a(18) =354508628a(19) =1221846856a(20) =4225644866a(21) =14659644348a(22) =51002664023a(23) =177909901566a(24) =622093882290