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)=7.

Terms

a(0) =1a(1) =1a(2) =7a(3) =7a(4) =7a(5) =56a(6) =105a(7) =154a(8) =546a(9) =1330a(10) =2800a(11) =7357a(12) =17745a(13) =52192a(14) =129171a(15) =330316a(16) =886809a(17) =2507337a(18) =6683558a(19) =17187443a(20) =46548096a(21) =130632390a(22) =352275462a(23) =938225351a(24) =2562724703a(25) =7217024738a(26) =19723748282a(27) =53440726780a(28) =146510223251a(29) =416055253418