a(n) = 2*(1+(-1)^n)/3 + 2*A010892(n-1).

Terms

a(0) =1a(1) =2a(2) =4a(3) =2a(4) =4a(5) =8a(6) =22a(7) =44a(8) =88a(9) =170a(10) =340a(11) =680a(12) =1366a(13) =2732a(14) =5464a(15) =10922a(16) =21844a(17) =43688a(18) =87382a(19) =174764a(20) =349528a(21) =699050a(22) =1398100a(23) =2796200a(24) =5592406a(25) =11184812a(26) =22369624a(27) =44739242