a(n) = 3*A000984(n) - 2.

Terms

a(0) =1a(1) =4a(2) =16a(3) =58a(4) =208a(5) =754a(6) =2770a(7) =10294a(8) =38608a(9) =145858a(10) =554266a(11) =2116294a(12) =8112466a(13) =31201798a(14) =120349798a(15) =465352558a(16) =1803241168a(17) =7000818658a(18) =27225405898a(19) =106035791398a(20) =413539586458