a(n) = (4^n + (-3)^n + 2^n + (-1)^n)/2.

Terms

a(0) =1a(1) =15a(2) =22a(3) =177a(4) =406a(5) =2445a(6) =7162a(7) =36177a(8) =121486a(9) =554325a(10) =2009602a(11) =8656377a(12) =32761366a(13) =136617405a(14) =529712842a(15) =2169039777a(16) =8525430046a(17) =34553579685a(18) =136858084882a(19) =551499730377