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

Terms

a(0) =1a(1) =1a(2) =6a(3) =6a(4) =6a(5) =42a(6) =78a(7) =114a(8) =366a(9) =870a(10) =1806a(11) =4470a(12) =10374a(13) =29490a(14) =70422a(15) =172050a(16) =443670a(17) =1203270a(18) =3079590a(19) =7634310a(20) =19822470a(21) =53513466a(22) =138825006a(23) =355353522a(24) =932324478a(25) =2521602582a(26) =6627329070a(27) =17259334422a(28) =45447231174a(29) =124012157154