a(n) = (-4 + (-2)^n + 2*3^(n+1))/3 - [n=0].

Terms

a(0) =0a(1) =4a(2) =18a(3) =50a(4) =166a(5) =474a(6) =1478a(7) =4330a(8) =13206a(9) =39194a(10) =118438a(11) =353610a(12) =1064246a(13) =3185914a(14) =9571398a(15) =28686890a(16) =86115286a(17) =258236634a(18) =774928358a(19) =2324348170a(20) =6973918326a(21) =20920007354a(22) =62763517318a(23) =188283561450a(24) =564864665366