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

Terms

a(0) =1a(1) =6a(2) =24a(3) =84a(4) =276a(5) =876a(6) =2724a(7) =8364a(8) =25476a(9) =77196a(10) =233124a(11) =702444a(12) =2113476a(13) =6352716a(14) =19082724a(15) =57297324a(16) =171990276a(17) =516167436a(18) =1548895524a(19) =4647473004a(20) =13943991876a(21) =41835121356a(22) =125511655524a(23) =376547549484