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Algebra Difficulty 8.7 Shortlist Find the answer

Let PP be a polynomial with integer coefficients such that P(0)=0P(0)=0 and
gcd(P(0),P(1),P(2),)=1.\gcd(P(0), P(1), P(2), \ldots ) = 1.
Show there are infinitely many nn such that
gcd(P(n)P(0),P(n+1)P(1),P(n+2)P(2),)=n.\gcd(P(n)- P(0), P(n+1)-P(1), P(n+2)-P(2), \ldots) = n.

A number or a short expression. Spacing and $ signs are ignored.

Solution

Let P P be a polynomial with integer coefficients such that P(0)=0 P(0) = 0 and
gcd(P(0),P(1),P(2),)=1. \gcd(P(0), P(1), P(2), \ldots ) = 1.
We need to show that there are infinitely many n n such that
gcd(P(n)P(0),P(n+1)P(1),P(n+2)P(2),)=n. \gcd(P(n) - P(0), P(n+1) - P(1), P(n+2) - P(2), \ldots) = n.

Consider the polynomial P(x)=xkQ(x) P(x) = x^k Q(x) , where Q(0)0 Q(0) \neq 0 . We claim that if n=pk n = p^k for any prime p p not dividing kQ(1)+Q(1) k Q(1) + Q'(1) , then n n will satisfy the conditions of the problem.

It is clear that pkP(pk+i)P(i) p^k \mid P(p^k + i) - P(i) for all i i , hence pkgcd(P(pk+i)P(0),P(pk+1)P(1),P(pk+2)P(2),) p^k \mid \gcd(P(p^k + i) - P(0), P(p^k + 1) - P(1), P(p^k + 2) - P(2), \ldots) . We now claim that pk+1P(pk+1)P(1) p^{k+1} \nmid P(p^k + 1) - P(1) . Let Q(x)=i=0mcixi Q(x) = \sum_{i=0}^m c_i x^i , then
P(pk+1)P(1)=(pk+1)kQ(pk+1)Q(1)=(i=0k(ki)(pk)i)Q(pk+1)Q(1)(kpk+1)Q(pk+1)Q(1)=kpkQ(pk+1)+(Q(pk+1)Q(1))=kpkQ(pk+1)+i=0mci((pk+1)i1i)=kpkQ(pk+1)+i=0mci(1+j=0i(ij)(pk)j)kpkQ(pk+1)+i=0micipk=pk(kQ(pk+1)+Q(1))(modpk+1). \begin{align*} P(p^k + 1) - P(1) &= (p^k + 1)^k Q(p^k + 1) - Q(1) \\ &= \left( \sum_{i=0}^k \binom{k}{i} (p^k)^i \right) Q(p^k + 1) - Q(1) \\ &\equiv (k p^k + 1) Q(p^k + 1) - Q(1) \\ &= k p^k Q(p^k + 1) + (Q(p^k + 1) - Q(1)) \\ &= k p^k Q(p^k + 1) + \sum_{i=0}^m c_i ((p^k + 1)^i - 1^i) \\ &= k p^k Q(p^k + 1) + \sum_{i=0}^m c_i \left(-1 + \sum_{j=0}^i \binom{i}{j} (p^k)^j \right) \\ &\equiv k p^k Q(p^k + 1) + \sum_{i=0}^m i c_i p^k \\ &= p^k (k Q(p^k + 1) + Q'(1)) \pmod{p^{k+1}}. \end{align*}
Thus,
P(pk+1)P(1)pk=kQ(pk+1)+Q(1)kQ(1)+Q(1)≢0(modp), \frac{P(p^k + 1) - P(1)}{p^k} = k Q(p^k + 1) + Q'(1) \equiv k Q(1) + Q'(1) \not\equiv 0 \pmod{p},
so pk p^k fully divides gcd(P(pk+i)P(0),P(pk+1)P(1),P(pk+2)P(2),) \gcd(P(p^k + i) - P(0), P(p^k + 1) - P(1), P(p^k + 2) - P(2), \ldots) .

To show that no other prime qp q \neq p divides each of P(pk)P(0),P(pk+1)P(1),P(pk+2)P(2), P(p^k) - P(0), P(p^k + 1) - P(1), P(p^k + 2) - P(2), \ldots , suppose for contradiction that qP(pk+i)P(i) q \mid P(p^k + i) - P(i) for all positive integers i i . Observing that qP(q+i)P(i) q \mid P(q + i) - P(i) , we find that P(i+apk+bq)P(i)(modq) P(i + ap^k + bq) \equiv P(i) \pmod{q} for all integers a a and b b . Since gcd(pk,q)=1 \gcd(p^k, q) = 1 , we can choose a a and b b such that apk+bq=1 ap^k + bq = 1 , yielding qP(i+1)P(i) q \mid P(i + 1) - P(i) for all i i . But qP(0)=0 q \mid P(0) = 0 , so qP(i) q \mid P(i) for all nonnegative i i , which contradicts the hypothesis.

Therefore, there are infinitely many n n such that
gcd(P(n)P(0),P(n+1)P(1),P(n+2)P(2),)=n. \gcd(P(n) - P(0), P(n + 1) - P(1), P(n + 2) - P(2), \ldots) = n.

The answer is: \boxed{\text{infinitely many } n}.

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