Number of nondecreasing integer sequences of length 18 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) =1761a(10) =3118a(11) =5855a(12) =9954a(13) =17513a(14) =29161a(15) =49089a(16) =78882a(17) =128692a(18) =201207a(19) =317716a(20) =486382a(21) =745954a(22) =1116059a(23) =1675856a(24) =2456983a(25) =3606019a(26) =5200369a(27) =7489188a(28) =10616424a(29) =15042972