a(n) = Sum_{k=0..10} binomial(20,k)*binomial(n,k).

Terms

a(0) =1a(1) =21a(2) =231a(3) =1771a(4) =10626a(5) =53130a(6) =230230a(7) =888030a(8) =3108105a(9) =10015005a(10) =30045015a(11) =84504355a(12) =223651350a(13) =558350430a(14) =1318250890a(15) =2952624906a(16) =6296642121a(17) =12834146941a(18) =25098124271a(19) =47262174531a(20) =85990654178a(21) =151631858378a(22) =259857912678a(23) =433877085278a(24) =707369215553