Maths Olympiad Prep

Library / /7 of 72

Algebra Difficulty 5.5 AIME, harder Prove it Vietnam

Does there exist a polynomial P(x)P(x) with integer coefficients such that P(1+23)=1+23P(1+\sqrt[3]{2}) = 1+\sqrt[3]{2} and P(1+5)=2+35P(1+\sqrt{5}) = 2+3\sqrt{5}?

Solution

Suppose that there exists such polynomial P(x)P(x). Let Q(x)=P(1+x)1Q(x) = P(1+x) - 1 then Q(x)Z[x]Q(x) \in \mathbb{Z}[x]. We have Q(23)=23Q(\sqrt[3]{2}) = \sqrt[3]{2} and Q(5)=1+35Q(\sqrt{5}) = 1 + 3\sqrt{5}.

Therefore, Q(x)xQ(x) - x has an irrational root 23\sqrt[3]{2}. Since x32x^3 - 2 is irreducible over Z[x]\mathbb{Z}[x] and has the same root 23\sqrt[3]{2}, we get x32Q(x)xx^3 - 2 \mid Q(x) - x. Then there exists R(x)R(x) with integer coefficients such that
Q(x)x=(x32)R(x). Q(x) - x = (x^3 - 2)R(x).
Because R(x)Z[x]R(x) \in \mathbb{Z}[x] so R(5)=a+b5R(\sqrt{5}) = a + b\sqrt{5} for some a,bZa, b \in \mathbb{Z}. Let x=5x = \sqrt{5} then:
1+35=(552)(a+b5)=25b2a+(5a2b)5, 1 + 3\sqrt{5} = (5\sqrt{5} - 2)(a + b\sqrt{5}) = 25b - 2a + (5a - 2b)\sqrt{5},
This implies 5a2b=35a - 2b = 3 and 2a=25b12a = 25b - 1, which has not a pair integer (a,b)(a,b) satisfying, a contradiction. So the answer is no.

Want a route through all this instead of an archive? The track puts 2,000 problems in a working order, from AMC 10 level to the IMO shortlist.

Source: MathNet, licensed CC-BY-4.0. Statement reproduced verbatim; metadata (topic, difficulty) added by this project.