a(0) = 2, a(1) = 3, a(n) = 4 * a(n-1) - a(n-2).

Terms

a(0) =2a(1) =3a(2) =10a(3) =37a(4) =138a(5) =515a(6) =1922a(7) =7173a(8) =26770a(9) =99907a(10) =372858a(11) =1391525a(12) =5193242a(13) =19381443a(14) =72332530a(15) =269948677a(16) =1007462178a(17) =3759900035a(18) =14032137962a(19) =52368651813a(20) =195442469290a(21) =729401225347