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Difference between revisions of "1993 IMO Problems/Problem 1"

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Q.E.D.
 
Q.E.D.
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== Solution 2 ==
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Let us assume the contrary. If <math>f(x)=p(x)q(x)</math>, then that is equivalent to <math>x^{n-1}(x+5)=p(x)q(x)-3</math>. Now we see that <math>p(0)q(0)=3</math> and <math>p(-5)q(-5)=3</math>. This implies that the constant terms of <math>p</math> and <math>q</math> are either <math>-1,-3</math> or <math>1,3</math>, up to permutation. Let <math>p(x)=a(x)+3</math> and <math>q(x)=b(x)+1</math>. The only possibility for <math>p(-5)q(-5)</math> to be equal to <math>3</math> is to make <math>a(-5)=b(-5)=0</math>. This is because if they are not zero they must be nonzero multiples of 5, and so <math>p(-5)q(-5)</math> must be larger than 3. So <math>p(x)-3=(x+5)A(x)</math> and <math>q(x)-1=(x+5)B(x)</math>. Now plug these back to  <math>x^{n-1}(x+5)=p(x)q(x)-3</math> we see that <cmath>x^{n-1}(x+5)=[(x+5)A(x)+3][(x+5)B(x)+1]-3</cmath>. This is equivalent to <cmath>(x+5)^2A(x)B(x)+(x+5)A(x)+3(x+5)B(x)=(x+5)x^{n-1}</cmath>. One of <math>A(x)</math> and <math>B(x)</math> must be 0. But then that implies one of <math>p</math> and <math>q</math> is less than degree 1. So the assumption must be false. The <math>(-1,-3)</math> case is similar.
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==Alternate Solution==
 
==Alternate Solution==

Revision as of 22:23, 28 September 2025

Problem

Let $f\left(x\right)=x^n+5x^{n-1}+3$, where $n>1$ is an integer. Prove that $f\left(x\right)$ cannot be expressed as the product of two non-constant polynomials with integer coefficients.

Solution

For the sake of contradiction, assume that $f\left(x\right)=g\left(x\right)h\left(x\right)$ for polynomials $g\left(x\right)$ and $h\left(x\right)$ in $\mathbb{R}$. Furthermore, let $g\left(x\right)=b_mx^m+b_{m-1}x^{m-1}+\ldots+b_1x+b_0$ with $b_i=0$ if $i>m$ and $h\left(x\right)=c_{n-m}x^{n-m}+c_{n-m-1}x^{n-m-1}+\ldots+c_1x+c_0$ with $c_i=0$ if $i>n-m$. This gives that $f\left(x\right)=\sum_{i=0}^{n}\left(\sum_{j=0}^{i}b_jc_{i-j}\right)x^i$.

We have that $3=b_0c_0$, or $3|b_0c_0$. WLOG, let $3|b_0$ (and thus $3\not|c_0$). Since $b_0c_1+b_1c_0=0$ and $3$ divides $b_0$ but not $c_0$, we need that $3|b_1$. We can keep on going up the chain until we get that $3|b_{n-2}$. Then, by equating coefficients once more, we get that $b_0c_{n-1}+b_1c_{n-2}+\ldots+b_{n-2}c_1+b_{n-1}c_0=5$. Taking the equation $\pmod3$ gives that $b_{n-1}c_0\equiv2\pmod3$. This implies that $b_{n-1}\neq0$. Thus, the degree of $g\left(x\right)$ is at least $n-1$. However, because $h\left(x\right)$ is a non-constant factor of $f\left(x\right)$, we have that the degree of $g\left(x\right)$ is at most $n-1$. Thus, the degree of $g\left(x\right)$ is $n-1$, implying that the degree of $h\left(x\right)$ is $1$.

From this fact, we have that there must exist an integer root of $f\left(x\right)$. However, when $x$ is an integer, $x^n + 5x^{n - 1} \equiv x^{n - 2}x(x + 1) \equiv 0 \pmod{2}$, meaning $f(x)$ is odd, so $x$ cannot be a root of $f$.

Hence, $f\left(x\right)$ cannot be expressed as $g\left(x\right)h\left(x\right)$ for polynomials $g\left(x\right)$ and $h\left(x\right)$ in $\mathbb{R}$. This means that $f\left(x\right)$ cannot be expressed as the product of two non-constant polynomials with integer coefficients.

Q.E.D.

Solution 2

Let us assume the contrary. If $f(x)=p(x)q(x)$, then that is equivalent to $x^{n-1}(x+5)=p(x)q(x)-3$. Now we see that $p(0)q(0)=3$ and $p(-5)q(-5)=3$. This implies that the constant terms of $p$ and $q$ are either $-1,-3$ or $1,3$, up to permutation. Let $p(x)=a(x)+3$ and $q(x)=b(x)+1$. The only possibility for $p(-5)q(-5)$ to be equal to $3$ is to make $a(-5)=b(-5)=0$. This is because if they are not zero they must be nonzero multiples of 5, and so $p(-5)q(-5)$ must be larger than 3. So $p(x)-3=(x+5)A(x)$ and $q(x)-1=(x+5)B(x)$. Now plug these back to $x^{n-1}(x+5)=p(x)q(x)-3$ we see that \[x^{n-1}(x+5)=[(x+5)A(x)+3][(x+5)B(x)+1]-3\]. This is equivalent to \[(x+5)^2A(x)B(x)+(x+5)A(x)+3(x+5)B(x)=(x+5)x^{n-1}\]. One of $A(x)$ and $B(x)$ must be 0. But then that implies one of $p$ and $q$ is less than degree 1. So the assumption must be false. The $(-1,-3)$ case is similar.


Alternate Solution

It’s actually trivial by Perron’s Criterion lol


Note: Quoting Perron's Criterion on the actual IMO will very likely result in a score in the set $\{0,1\}$, since it was not a well-known result back then.

See Also

1993 IMO (Problems) • Resources
Preceded by
First Question 		1 • 2 • 3 • 4 • 5 • 6 Followed by
Problem 2
All IMO Problems and Solutions
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