G.f. A(x) satisfies A(x) = 1/(1 - x*A(x)/(1 - x^3)).

Terms

a(0) =1a(1) =1a(2) =2a(3) =5a(4) =15a(5) =46a(6) =147a(7) =486a(8) =1646a(9) =5684a(10) =19940a(11) =70864a(12) =254592a(13) =923153a(14) =3374046a(15) =12417246a(16) =45975677a(17) =171141378a(18) =640105278a(19) =2404375805a(20) =9066188052a(21) =34305301482a(22) =130219435385a(23) =495735347502