If n mod 2 = 0 then (3^(n+3)-19)/8 else (3^(n+3)-1)/8.

Terms

a(0) =1a(1) =10a(2) =28a(3) =91a(4) =271a(5) =820a(6) =2458a(7) =7381a(8) =22141a(9) =66430a(10) =199288a(11) =597871a(12) =1793611a(13) =5380840a(14) =16142518a(15) =48427561a(16) =145282681a(17) =435848050a(18) =1307544148a(19) =3922632451a(20) =11767897351a(21) =35303692060a(22) =105911076178a(23) =317733228541a(24) =953199685621