a(n) = 4^n - 2*3^n.

Terms

a(0) =-1a(1) =-2a(2) =-2a(3) =10a(4) =94a(5) =538a(6) =2638a(7) =12010a(8) =52414a(9) =222778a(10) =930478a(11) =3840010a(12) =15714334a(13) =63920218a(14) =258869518a(15) =1045044010a(16) =4208873854a(17) =16921588858a(18) =67944635758a(19) =272553384010