a(n) = A002526(n+1) - A002527(n+1).

Terms

a(0) =0a(1) =0a(2) =0a(3) =6a(4) =18a(5) =46a(6) =115a(7) =374a(8) =1204a(9) =3752a(10) =11300a(11) =34324a(12) =105124a(13) =322989a(14) =989692a(15) =3028484a(16) =9267328a(17) =28374898a(18) =86891022a(19) =266058106a(20) =814585879a(21) =2494006074a(22) =7636057864a(23) =23380074400a(24) =71584762200a(25) =219176102664a(26) =671066472872