Processing math: 100%

Monday, September 9, 2019

Integer polynomial property

P is a polynomial with integer coefficients. Show that if a is an integer such that

P(P(P(a)))=a


then

P(a)=a



Solution Sketch:


This uses the fact that P(x)P(y) is divisible by xy to get a cyclic chain of divisibility conditions implying each one in the chain is ±1 times the others. Some assumptions like P(a)>a etc lead to contradictions.


No comments:

Post a Comment