Number of nondecreasing integer sequences of length 40 with sum zero and sum of absolute values 2n.

Terms

a(0) =1a(1) =4a(2) =9a(3) =25a(4) =49a(5) =121a(6) =225a(7) =484a(8) =900a(9) =1764a(10) =3136a(11) =5929a(12) =10201a(13) =18225a(14) =30976a(15) =53361a(16) =88209a(17) =148225a(18) =240100a(19) =393129a(20) =627261a(21) =1003986a(22) =1574951a(23) =2480376a(24) =3833030a(25) =5932129a(26) =9055198a(27) =13812005a(28) =20813458a(29) =31349531