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

Terms

a(0) =0a(1) =2a(2) =10a(3) =42a(4) =170a(5) =682a(6) =2730a(7) =10922a(8) =43690a(9) =174762a(10) =699050a(11) =2796202a(12) =11184810a(13) =44739242a(14) =178956970a(15) =715827882a(16) =2863311530a(17) =11453246122a(18) =45812984490a(19) =183251937962a(20) =733007751850