a(n) = a(n-1) + 3*a(n-2) if n is even, otherwise a(n) = 3*a(n-1) + a(n-2), a(0)=0, a(1)=1.

Terms

a(0) =0a(1) =1a(2) =1a(3) =4a(4) =7a(5) =25a(6) =46a(7) =163a(8) =301a(9) =1066a(10) =1969a(11) =6973a(12) =12880a(13) =45613a(14) =84253a(15) =298372a(16) =551131a(17) =1951765a(18) =3605158a(19) =12767239a(20) =23582713a(21) =83515378a(22) =154263517a(23) =546305929a(24) =1009096480a(25) =3573595369a(26) =6600884809a(27) =23376249796a(28) =43178904223a(29) =152912962465