Constant term in expansion of (1/2) * Product_{k=-n..n} (1 + x^k).

Terms

a(0) =1a(1) =2a(2) =4a(3) =10a(4) =26a(5) =76a(6) =236a(7) =760a(8) =2522a(9) =8556a(10) =29504a(11) =103130a(12) =364548a(13) =1300820a(14) =4679472a(15) =16952162a(16) =61790442a(17) =226451036a(18) =833918840a(19) =3084255128a(20) =11451630044a(21) =42669225172a(22) =159497648600a(23) =597950875256