theorem Th20: :: LAPLACE:20

for n, i, j being Nat st i in Seg (n + 1) & j in Seg (n + 1) holds
for P being set st P = { p1 where p1 is Element of Permutations (n + 1) : p1 . i = j } holds
ex Proj being Function of P,(Permutations n) st
( Proj is bijective & ( for q1 being Element of Permutations (n + 1) st q1 . i = j holds
Proj . q1 = Rem (q1,i) ) )