2-adic factorial function.

Terms

a(0) =1a(1) =-1a(2) =1a(3) =-3a(4) =3a(5) =-15a(6) =15a(7) =-105a(8) =105a(9) =-945a(10) =945a(11) =-10395a(12) =10395a(13) =-135135a(14) =135135a(15) =-2027025a(16) =2027025a(17) =-34459425a(18) =34459425a(19) =-654729075a(20) =654729075a(21) =-13749310575a(22) =13749310575a(23) =-316234143225a(24) =316234143225