theorem Th8: :: CARDFIN2:8

for s being non empty finite set ex c being Real st
( c in ].(- 1),0.[ & (card (derangements s)) - (((card s) !) / number_e) = - (((card s) !) * (((exp_R c) * ((- 1) |^ ((card s) + 1))) / (((card s) + 1) !))) )