Number of nondecreasing integer sequences of length 21 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) =3135a(11) =5921a(12) =10163a(13) =18086a(14) =30544a(15) =52168a(16) =85231a(17) =141308a(18) =225085a(19) =361916a(20) =565231a(21) =884209a(22) =1351340a(23) =2071388a(24) =3105033a(25) =4656911a(26) =6869566a(27) =10115584a(28) =14675797a(29) =21268848