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Algebra Difficulty 6.9 National olympiad Prove it Saudi Arabia

Does there exist a polynomial P(x)P(x) with integral coefficients such that

1. P(253+53)=220253+28453P(\sqrt[3]{25}+\sqrt[3]{5})=220 \sqrt[3]{25}+284 \sqrt[3]{5} ?

2. P(253+53)=1184253+121053P(\sqrt[3]{25}+\sqrt[3]{5})=1184 \sqrt[3]{25}+1210 \sqrt[3]{5} ?

Solution

First, we shall prove two following lemmas:

Lemma 1. If aa is an integer number that is not a perfect cube, and m,n,pm, n, p are integer numbers such that m+na3+pa23=0m+n \sqrt[3]{a}+p \sqrt[3]{a^{2}}=0, then
m=n=p=0. m=n=p=0 .
Proof of Lemma 1. Since m+na3+pa23=0m+n \sqrt[3]{a}+p \sqrt[3]{a^{2}}=0, then a3\sqrt[3]{a} is the root of P(x)=px2+nx+mP(x)=p x^{2}+n x+m with integer coefficients.
Consider the polynomial Q(x)=x3aQ(x)=x^{3}-a, then a3\sqrt[3]{a} is also a root of Q(x)Q(x). Assume that Q(x)Q(x) is reducible, then one of the factors must have degree 1, that is Q(x)Q(x) has a rational root. But we know that a3\sqrt[3]{a} is irrational since aa is not a perfect cube (by assumption). Hence, Q(x)Q(x) is irreducible.
In other words, Q(x)Q(x) is the minimal polynomial of a3\sqrt[3]{a}, and this leads to P(x)=0P(x)=0, or m=n=p=0m=n=p=0.

Lemma 2. Given that P(x)P(x) is a polynomial with integer coefficients, and aa is an integer such that a3\sqrt[3]{a} is not an integer. If
P(a23+a3)=m+na3+pa23 P\left(\sqrt[3]{a^{2}}+\sqrt[3]{a}\right)=m+n \sqrt[3]{a}+p \sqrt[3]{a^{2}}
then a1npa-1 \mid n-p.

Proof of Lemma 2. We will prove by induction on the degree of P(x)P(x) which is denoted by kk.
For k=1k=1, it is trivial.
Assume that the statement is true for k=tk=t, consider P(x)P(x) of degree t+1t+1, we write P(x)=xQ(x)+sP(x)=x Q(x)+s, with Q(x)Q(x) of degree tt and s=P(0)s=P(0) is an integer.
Assume that Q(a23+a3)=d+ea3+fa23Q\left(\sqrt[3]{a^{2}}+\sqrt[3]{a}\right)=d+e \sqrt[3]{a}+f \sqrt[3]{a^{2}}, then by induction hypothesis a1efa-1 \mid e-f. This gives
P(a23+a3)=(a23+a3)(d+ea3+fa23)+s=s+ea+fa+(d+fa)a3+(d+e)a23 \begin{aligned} P\left(\sqrt[3]{a^{2}}+\sqrt[3]{a}\right) & =\left(\sqrt[3]{a^{2}}+\sqrt[3]{a}\right)\left(d+e \sqrt[3]{a}+f \sqrt[3]{a^{2}}\right)+s \\ & =s+e a+f a+(d+f a) \sqrt[3]{a}+(d+e) \sqrt[3]{a^{2}} \end{aligned}
Hence, (d+fa)(d+e)=fae=(faf)+(fe)(d+f a)-(d+e)=f a-e=(f a-f)+(f-e), which is a multiple of a1a-1.

Now we turn to the problem.

For part a), we show that P(x)P(x) does exist, for example we choose P(x)=ux2+vx+wP(x)= u x^{2}+v x+w, where u,v,wu, v, w must satisfy
u+v=220,  5u+v=284,  10u+w=0. u+v=220,\;5 u+v=284,\;10 u+w=0 .
That is, P(x)=16x2+204x160P(x)=16 x^{2}+204 x-160.

For part b), since 12101184=261210-1184=26, which is not a multiple of 44, in this case there is no P(x)P(x) satisfying the condition of the problem.

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