a(n) = 3^n + (4^n - 3^n) * (d(n) - 3), where d(n) = A000005(n).

Terms

a(0) =1a(1) =2a(2) =-10a(3) =81a(4) =-538a(5) =4096a(6) =-12010a(7) =65536a(8) =19683a(9) =1048576a(10) =-3840010a(11) =49268766a(12) =-63920218a(13) =268435456a(14) =1073741824a(15) =8546887871a(16) =-16921588858a(17) =205383589230a(18) =-272553384010a(24) =847288609443