Given a set a(1), a(2), a(3)...a(n), n>1.
Let P0 be the number of permutations of n elements, in which no element a(i) is in place i.
Let P1 be the number of permutations of n elements, in which exactly one element is in its original place i.
Show that ABS(P0-P1)=1.
When is P0-P1 positive?