a(n) = 1 + n + n*[n/2] + n*[n/2]*[n/3] + n*[n/2]*[n/3]*[n/4] +... where [x]=floor(x).

Terms

a(0) =1a(1) =2a(2) =5a(3) =10a(4) =29a(5) =46a(6) =169a(7) =239a(8) =745a(9) =1450a(10) =4111a(11) =5182a(12) =27157a(13) =33164a(14) =84001a(15) =186496a(16) =610065a(17) =713474a(18) =3061009a(19) =3526553a(20) =13783421a(21) =27380452a(22) =63264389a(23) =71240523a(24) =444872761a(25) =620729126a(26) =1400231613a(27) =2615011102a(28) =9094701085a(29) =10008828958