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

Terms

a(0) =1a(1) =0a(2) =6a(3) =2a(4) =26a(5) =10a(6) =106a(7) =42a(8) =426a(9) =170a(10) =1706a(11) =682a(12) =6826a(13) =2730a(14) =27306a(15) =10922a(16) =109226a(17) =43690a(18) =436906a(19) =174762a(20) =1747626a(21) =699050a(22) =6990506a(23) =2796202a(24) =27962026a(25) =11184810a(26) =111848106a(27) =44739242a(28) =447392426a(29) =178956970