Binomial coefficient C(21,n).

Terms

a(0) =1a(1) =21a(2) =210a(3) =1330a(4) =5985a(5) =20349a(6) =54264a(7) =116280a(8) =203490a(9) =293930a(10) =352716a(11) =352716a(12) =293930a(13) =203490a(14) =116280a(15) =54264a(16) =20349a(17) =5985a(18) =1330a(19) =210a(20) =21a(21) =1