a(n) = floor((concatenation of first n natural numbers)/ (n!)).

Terms

a(0) =1a(1) =6a(2) =20a(3) =51a(4) =102a(5) =171a(6) =244a(7) =306a(8) =340a(9) =3402a(10) =30928a(11) =257737a(12) =1982597a(13) =14161414a(14) =94409428a(15) =590058925a(16) =3470934853a(17) =19282971406a(18) =101489323191a(19) =507446615959