(1/1 - 1/3 + 1/6 + ... + d/C(n+1,2))*LCM{1,3,6,...,C(n+1,2)}, where d = (-1)^n.

Terms

a(0) =1a(1) =2a(2) =5a(3) =22a(4) =24a(5) =158a(6) =331a(7) =958a(8) =986a(9) =10594a(10) =10804a(11) =138142a(12) =140122a(13) =138406a(14) =279815a(15) =4711810a(16) =4751850a(17) =89604470a(18) =90217082a(19) =89662814a(20) =90166694a(21) =2063252482a(22) =2072952172a(23) =10320142286a(24) =10361328662a(25) =30969579386a(26) =31075814086a(27) =898330271594a(28) =901007386034