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 x−y 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.
P(P(P(a)))=a
then
P(a)=a
Solution Sketch:
This uses the fact that P(x)−P(y) is divisible by x−y 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.
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