a(n) is the constant term in expansion of Product_{k=1..n} (x^(k*(k+1)/2) + 1 + 1/x^(k*(k+1)/2)).

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =3a(5) =5a(6) =11a(7) =27a(8) =61a(9) =133a(10) =311a(11) =761a(12) =1839a(13) =4575a(14) =11573a(15) =29641a(16) =76487a(17) =199617a(18) =524067a(19) =1384697a(20) =3681069a(21) =9841217a(22) =26437741a(23) =71369101a(24) =193496241a(25) =526685793a(26) =1438816755a(27) =3944034221a(28) =10845006963a(29) =29908325821