a(n) = 3*a(n-1) - a(n-2) - a(n-3) + 12.

Terms

a(0) =0a(1) =1a(2) =13a(3) =50a(4) =148a(5) =393a(6) =993a(7) =2450a(8) =5976a(9) =14497a(10) =35077a(11) =84770a(12) =204748a(13) =494409a(14) =1193721a(15) =2882018a(16) =6957936a(17) =16798081a(18) =40554301a(19) =97906898a(20) =236368324a(21) =570643785a(22) =1377656145a(23) =3325956338a(24) =8029569096a(25) =19385094817