a(n) = Sum_{i=1..floor((n+1)/4)} a(2*i-1) * a(n-2*i+1), with a(1)=a(2)=1 and a(3)=6.

Terms

a(0) =1a(1) =1a(2) =6a(3) =6a(4) =6a(5) =6a(6) =42a(7) =78a(8) =114a(9) =366a(10) =870a(11) =1806a(12) =4470a(13) =10374a(14) =26682a(15) =63510a(16) =151962a(17) =375414a(18) =936294a(19) =2295078a(20) =5570214a(21) =13729974a(22) =35174202a(23) =87526302a(24) =215709522a(25) =540037374a(26) =1388390982a(27) =3485855310a(28) =8700035046a(29) =21930804870