The sequence b[n] defined in A126940.

Terms

a(0) =0a(1) =2a(2) =4a(3) =12a(4) =40a(5) =128a(6) =404a(7) =1282a(8) =4076a(9) =12946a(10) =41110a(11) =130570a(12) =414708a(13) =1317126a(14) =4183250a(15) =13286248a(16) =42197850a(17) =134022604a(18) =425663006a(19) =1351928684a(20) =4293798244a(21) =13637334278a(22) =43312907878a(23) =137564127106a(24) =436911072754