a(0)=a(1)=1, a(2)=2; thereafter a(n) = 3*a(n-1) + 3*2^(n-3) - 2.

Terms

a(0) =1a(1) =1a(2) =2a(3) =7a(4) =25a(5) =85a(6) =277a(7) =877a(8) =2725a(9) =8365a(10) =25477a(11) =77197a(12) =233125a(13) =702445a(14) =2113477a(15) =6352717a(16) =19082725a(17) =57297325a(18) =171990277a(19) =516167437a(20) =1548895525a(21) =4647473005a(22) =13943991877a(23) =41835121357a(24) =125511655525a(25) =376547549485