-670438943970domain: ZAppears in sequencesa(n) = 10*9*8*7*6*5*4*3*2*1 - 20*19*18*17*16*15*14*13*12*11 + 30*19*18*17*16*15*14*13*12*11 - ... + (up to the n-th term).at n=20A319893