Second line of 4-wave sequence A038197.

Terms

a(0) =1a(1) =3a(2) =7a(3) =26a(4) =56a(5) =216a(6) =462a(7) =1791a(8) =3828a(9) =14849a(10) =31735a(11) =123111a(12) =263108a(13) =1020696a(14) =2181389a(15) =8462447a(16) =18085587a(17) =70160958a(18) =149945056a(19) =581694636a(20) =1243173370a(21) =4822748423a(22) =10306975580a(23) =39984728949a(24) =85453685055a(25) =331507764639a(26) =708484485384