For each positive integer $n$, denote by $\omega(n)$ the number of distinct prime divisors of $n$ (for example, $\omega(1)=0$ and $\omega(12)=2$). Find all polynomials $P(x)$ with integer coefficients, such that whenever $n$ is a positive integer satisfying $\omega(n)>2023^{2023}$, then $P(n)$ is also a positive integer with \[\omega(n)\ge\omega(P(n)).\] Greece (Minos Margaritis - Iasonas Prodromidis)
Problem
Source: BMO 2023 Problem 3
Tags: number theory, BMO 2023, gauss lemma
10.05.2023 20:44
@above $ -x^m$ isn’t a solution, because the problem statement requires $P(n)$ to be a positive integer whenever $n$ satisfies $\omega(n)>2023^2023$.
10.05.2023 22:12
@2above I might be misunderstanding your solution, but are you sure you didn't solve the problem for $\tau$ instead of $\omega$? I don't see how this power of $2$ will appear otherwise.
10.05.2023 22:49
I think this is true Let $P(x)=a_vx^v+...+a_1x+a_0$. If $a_0=0$ let $P(x)=x^aQ(X)$ .Then for every $x$ we have that $Q(x)$ has the same primes as $x$.Let $Q(x)=b_sx^s+..+b_1x+b_0$ with $b_0$ different from $0$ select a prime $p$ with $p$ does not divese $b_0$ but $p|Q(x)$ then $p|x$ so $p|b_0$ contradiction so $Q(x)=+-1$. If $a_0$ different from $0$. Let $p_1,..,p_m$ be different primes who doesn't device $a_0$ and let $p_i|P(x_i)$ then $p_i$ doesn't device $x_i$ so by Chinese remainder Theorym and Direclert Theorym we can find $prime=p=x_i(mod p_i)$ Now let $r_1,r_2,...,r_{2023^{2023}}$ be different primes with $r_i=1(modp_1*..*p_m)$. If we take now $n=p*r_1*...*r_{2023^{2023}}$ contradiction by taking large $m$ So all the solution are: $P(x)=x^m$,$P(x)=c$ with $\omega(c)<=2023^{2023}+1$ and $c>0$
10.05.2023 22:57
Did anyone find a solution without Dirichlet's Theorem?
10.05.2023 23:29
Similar to above but likely easier to parse. Let $N=2023^{2023}$. We claim either (a) there is an $m\in\mathbb{N}$ such that $P(x)=x^m$ for all $x$ or (b) there is a $c\in\mathbb{N}$ with $\omega(c)\le N+1$ such that $P(x)=c$ for all $x$. The case $P$ is constant is clear, hence assume $P$ is non-constant. Let $P(x)=\alpha x^m Q(x)$ with $Q(0)\ne 0$. If $Q$ is constant, then $Q$ must clearly be one: otherwise if $Q=c$ with $|c|>1$ then by choosing primes $p_1<\cdots<p_{N+1}$ and letting $n=\prod_{1\le i\le N+1}p_i$, we arrive at a contradiction. Now assume $Q$ is non-constant, set $Q(x) = \alpha_d x^d+\cdots+\alpha_1 x+\alpha_0$ with $\alpha_d,\alpha_0\ne 0$. It is well-known that the set tof prime divisors of $Q$ is infinite. Let $q_1,\dots,q_T$ be distinct primes larger than $|\alpha_0|$ and $T\gg N$. Then there exists $a_i\ne 0$, $1\le i\le T$, such that $q_i\mid Q(a_i)$. Now let $p_1,\dots,p_{N+1}$ be distinct primes such that $p_1\equiv a_i\pmod{q_i}$ for $1\le i\le T$ and $p_j\equiv 1\pmod{q_1\cdots q_T}$ for $2\le j\le N+1$. Clearly such primes exist due to CRT + Dirichlet. We then let $n=p_1\cdots p_{N+1}$, so that $\omega(n)=N+1$, whereas $P(n)\equiv P(a_i)\equiv 0\pmod{q_i}$ so that $\omega(P(n))\ge T\gg N$, a contradiction.
10.05.2023 23:44
Basically the same solution as the two above.
10.05.2023 23:58
The answer is $P(x)=x^n$ or $P(x)=c$ where $\omega(c)\leq 2023^{2023}$ which clearly work. It is trivial to see that the given solutions are the only constant ones, as well as the only solutions of the form $cx^n$, henceforth suppose that $P$ has some factor which is not a power of $x$ (or constant). Let $N$ be the product of the first $2023^{2023}+1$ primes. First we prove that $x \nmid P(x)$. Suppose otherwise and let $P(x)=x^kR(x)$ where $k>0$ and $x \nmid R(x)$, so by Bezout's Lemma there exists some integer $M$ such that $\gcd(n, R(n))\leq M$ for all $n \in \mathbb{Z}^+$. On the other hand, by Schur's Lemma there exist arbitrarily large prime divisors of $R(Nx)$ (viewing this as a polynomial in $x$), so by picking $x$ such that $R(Nx)$ has a prime divisor $p>\max\{M,N\}$, so $p \mid P(Nx)$ but not $p \mid (Nx)^k$, so $\omega(P(Nx))>\omega(Nx)$ since every prime divisor of Nx is still a prime divisor of $P(Nx)$. Now we consider $x \nmid P(x)$ only. Let $Q(x)=P(Nx)$ so $x \nmid Q(x)$. By Schur's theorem we can find $2023^{2023}+3$ primes $p_1,p_2,\ldots$ not dividing $P(0) \neq 0$ and integers $a_1,a_2,\ldots$ such that $x \equiv a_i \pmod{p_i} \implies p_i \mid P(x)$, so we actually have $p_i \nmid a_i$ for all $i$. By the Chinese Remainder Theorem and Dirichlet, we can find some prime $P$ satisfying $P \equiv a_i \pmod{p_i}$ for all $i$, in which case $\omega(P(NP)) \geq 2023^{2023}+2$, but $\omega(NP)=2023^{2023}+2$: contradiction. Therefore no other polynomials work. $\blacksquare$ Remark: Kind of cute but also not particularly interesting. To some extent the entire problem should be routine to an experienced contestant, since I feel like this idea is simply the first thing you think of.
11.05.2023 00:01
Let $P(x)=a_mx^m+a_{m-1}x^{m-1}+\dots+a_0$. Since $P(n)$ needs to be a positive integer for arbitrarily large positive integers $n$, we must have that $a_m>0$, which also implies that $P\not\equiv 0$. If $m=0$, then $P{}$ is a constant polynomial; let $P(x)=c$. Since we must have $\omega(n)>2023^{2023}$ for the given inequality to apply, for $P{}$ to satisfy the given condition it is necessary and sufficient that $\omega(c)\leq 2023^{2023}+1$. Else, if $P(x)=a_mx^m, m\ge 1$, i.e., all coefficients except $a_m$ are zero, then by choosing $n$ satisfying the conditions and coprime to $a_m$, we get that we need to have $$\omega(n)\ge \omega(P(n))=\omega(a_mn^m)=\omega(a_m)+\omega(n^m)=\omega(a_m)+\omega(n) \rightarrow \omega(a_m)\le 0\rightarrow \omega(a_m)=0$$Hence, we must have that $a_m=1$, i.e. $P(x)=x^m$, which is easily seen to also be a solution to the problem. Now, suppose that there is some $m>t\ge 0$ such that $a_t\neq 0$ and take $t$ to be minimal. If $t\neq 0$, then $P(x)=x^tQ(x)$, where $Q(x)=a_mx^{m-t}+\dots+a_t$. We choose a large enough $n$ such that $Q(n)>1$ and $n{}$ is coprime to $a_t$, which means that $n{}$ is also coprime to $Q(n)$, and we have $\omega(n)\ge \omega(P(n))=\omega(n^t)+\omega(Q(n))\ge \omega(n)+1$, impossible. Thus, we must have $t=0$. To finish off the problem, we have the following lemma: Lemma: Let $k\in \mathbb{N}$ be fixed, and $P$ be a non-constant polynomial such that $P(0)\neq 0$. Then, there is some $n\in\mathbb{N}$ such that $\omega(n)=k$ and $\omega(P(n))$ is arbitrarily large. The lemma evidently finishes the problem, since we can set $k=2023^{2023}+1$ to see that all polynomials of such type cannot satisfy the given conditions.
**Note: Dirichletless
11.05.2023 14:41
My solution : The easy cases are when P si constant or $deg P \ge1$ and P(0)=0 which yeld the constant polynomial satisfying the relation and $x^d$ for some d So suppose deg P>=1 and P(0) is not 0 From Schur there exists an infinite number of primes $p_i$ which divideas some $P(n_i)$ and $p_i$ and P(0) are coprime. Choose k large and let $C=2023^{2023}$ Let's consider the system of congruences $X \equiv n_1 \pmod{p_1}$ $X \equiv n_2 \pmod{p_2}$ ........... $X \equiv n_k \pmod{p_k}$ and by CRT we get that $X \equiv T \pmod{p_1...p_k}$ and it is easy to see that T and $p_1..p_k$ are coprime so by Dirichlet there exists an infinite number of primes Take $n = q_1^{\alpha_1}q_2^{\alpha_2}\cdots q_(C+1)^{\alpha_(C+1)}$ so $\omega(n)>C$ so \[\omega(n)\ge\omega(P(n)).\]($q_1$ etc are primes from Dirichlet) But then it is easy that if we let ${\alpha_1 + \alpha_2 + \dots + \alpha_k} \equiv 1 \pmod{(p_1-1)...(p_k-1)}$then P(n) is divisible by $p_i$ so $\omega(P(n))>=k$ which is a contradiction because by hyphotesis we get that C+1>=k but k is large. So the only polynomials are $x^d$ and c
11.05.2023 15:05
Kinda cringe that this problem is either you know Dirichlet's theorem and u get 10 or you don't and get 0. I know Dirichlet is common knowledge but it still doesn't seem fair.
11.05.2023 15:23
Complete_quadrilateral wrote: Kinda cringe that this problem is either you know Dirichlet's theorem and u get 10 or you don't and get 0. I know Dirichlet is common knowledge but it still doesn't seem fair. Completely false. I didn’t use Dirichlet (actually, no theorem at all) and got a 10.
11.05.2023 19:24
A solution by bootstraping powers of a fixed number: Assume $P(0)\neq 0$ Let $M=2023^{2023}$. Select $q_{i}$ $i=1,...,M+1$ primes and $p$ a prime different from the aforementioned and pick $c$ such that: $c\equiv x_{p} \pmod p$ where $p\mid P(x_{p})$ ($x_{p}$ exists by Schur's theorem) $c\equiv 0 \pmod q_{i}$ By CRT there exists such c. Now we construct the following sequence: $a_{1}=1$, $a_{n+1}=a_{n}+\lambda\prod_{i=1}^{n} (p_{i}-1)$ for several primes and some $\lambda$ specified below (that satisfy: $p_{i}$ divides $P(c^{a_{n}})$: $p_{1}=p$. On the n-th step we select $\lambda$ and $p_{n}$ Note that $P(c^{a_{n}})\equiv P(c^{a_{n-1}})\equiv 0 \pmod p_{i}$ for all $i=1,2,...,n-1$. Claim: $P(c^{x+\lambda y})$ where $y=\prod_{i=1}^{n-1} (p_{i}-1)$, $x=a_{n-1}$ has infinitely many prime divisors when $\lambda$ runs through $N$. Proof: Assume that's not the case. Call a prime $q$ "scary" if and only if $q\mid P(c^{x+\lambda y})$ for some $\lambda$. There are finitely many scary primes. For each scary prime $q$ not dividing c, set $m_{q}=U_{q}(P(c^{x}))$ and set $\lambda \to \lambda \prod_{q scary}(\phi(q^{m_{q}+1}))=\lambda s$. Then $P(c^{x+\lambda sy})\equiv P(c^{x})\neq 0 \pmod q^{m_{q}+1}$ hence $U_{q}P(c^{x+\lambda sy})\leq m_{q}$, hence upper bounded. If $q\mid c$ then $q\mid P(c^{sth})\equiv c^{sth}+P(0)$, and when sth is large enough, $U_{q}(c^{sth})=U_{q}(P(0))$ hence upper bounded. Setting for brevity $Y=\prod_{q scary} (\phi(q^{m+1}))(\prod_{i=1}^{n-1} (p_{i}-1))$ it suffices to prove that $P(c^{x+\lambda Y})$ has infinitely many prime divisors, to reach a contradiction. So, all scary primes have upper bounded p-adic valuations when considering the subsequence of $\lambda$. Hence, since $P(c^{x+\lambda Y})$ grows without bound, it should have infinitely many primes dividing some of its terms and the lemma is proven. Select now a prime $P\mid P(c^{x+\lambda_{P} y})$ and set $a_{n}=a_{n-1}+\lambda_{P}\prod_{i=1}^{n-1} (p_{i}-1)$ and $p_{n}=P$. Note that $\omega(c^{a_{n}})$ stays constant and $\omega(P(c^{a_{n}}))$ grows without bound, hence we derive a contradiction. So the only solutions are $P(x)=x^{d}$ and $P(x)=c$.
11.05.2023 19:45
oVlad wrote: Complete_quadrilateral wrote: Kinda cringe that this problem is either you know Dirichlet's theorem and u get 10 or you don't and get 0. I know Dirichlet is common knowledge but it still doesn't seem fair. Completely false. I didn’t use Dirichlet (actually, no theorem at all) and got a 10. True but at least for me, i would never be able to solve it if I didn't know Dirichlet and i solved it in less that a hour cus i knew it. It still is a huge disadvantage if you don't know it since most of the solutions (on aops and i think on competition as well) use it.
11.05.2023 19:47
oVlad wrote: Complete_quadrilateral wrote: Kinda cringe that this problem is either you know Dirichlet's theorem and u get 10 or you don't and get 0. I know Dirichlet is common knowledge but it still doesn't seem fair. Completely false. I didn’t use Dirichlet (actually, no theorem at all) and got a 10. Also could u please post your solution I would like to see it.
12.05.2023 03:11
Let $P(n)=n^kQ(n)$ where $Q(0)\neq 0$. Let $p$ be a product of $2023^{2023}+1$ primes relatively prime to $Q(0)$. Then, if $Q$ is nonconstant, then by Schur's Theorem, there exists infinitely many primes dividing $Q(n)$ for some $n$, so there exists $m$ such that $Q(m)$ is divisible by a product of $2023^{2024}$ primes $q_i$ relatively prime to $p$ and $Q(0)$ with $\gcd(m,q_i)=1$ and $m\mid p$. Let $c=q_1q_2\ldots q_{2023^{2024}}$. Then, by Dirichlet's Theorem, there exists infinitely many primes of the form $cx+\frac mp$, so $\omega(pcx+m)=2023^{2023}+2$ and $\omega(P(pcx+m))\geq 2023^{2024}$, contradiction. Therefore, $Q$ is constant. If $k\geq1$ and $Q(n)>1$, then if $\gcd(n,Q(n))=1$, then $\omega(P(n))\geq\omega(n)+1$, contradiction. Therefore, we get either $P(n)=C$ for some $C$ satisfying $\omega(C)\leq2023^{2023}$ or $P(n)=n^k$, which both work.
13.05.2023 09:57
Change $P(x)$ to $f(x)$ and assume it is non constant (for constant polynomial any constant $c$ with $\omega(c) \le 2023^{2023}$ works. Suppose that $f(0) \neq 0$ and let $M$ be a sufficiently large natural. Step 1: Arbitrary primes dividing the polynomial. Start with some prime $p_1$ where $p_1 \mid f(x_1)$ and $\gcd(x_1,p_1)=1$. Choose some $p_2 \neq p_1$ where $p_2 \mid f(x_1 + kp_1)$ (treat this as a polynomial in $k$, by Dirichlet we can select $x_2 = x_1 + kp_1$ as a prime and by Schur we can guarantee a $p_2$) such that $\gcd(p_2, x_2)=1$, so $p_1p_2 \mid f(x_2)$, again $p_1p_2 \mid f(x_2 + kp_1p_2)$, repeat the process to get to a point where we have an $x$ such that $P=\displaystyle\prod_{i=1}^M p_i \mid f(x)$ and $\gcd(P,x)=1$. Step 2: Construct more primes till you get a contradiction. Choose primes $c_i \equiv 1\pmod P$ for $1\le i\le M$ and let $C=\displaystyle\prod_{i=1}^M c_i$. Note that $f(Cr) \equiv f(r)\pmod{p_i}$ and also note that the set of primes dividing $f(kP+x)$ is infinite by Schur (treating this as a polynomial in $x$ as we can construct different $x$ each time through the above process) there we can guarantee the existence of more primes (say $a,b$) such that $ab \mid f(kP+x)$, finally choose $r=kP+x$ as a prime, then $abP \mid f(Cr)$. \[M+1 = \omega(Cr) \ge \omega(f(Cr)) \ge M + 1 + 1\]a contradiction. Step 3: The finish. Write $f(x)=x^ng(x)$ where $g(0) \neq 0$, suppose that $g$ is non constant then we can apply Schur and pretty much repeat the entire thing above to get a contradiction. So finally $f(x)=mx^n$, if there is some prime $p$ dividing $m$ then easily we can get a contradiction by selecting $x$ to be some number with $\nu_p(x)=0$, thus the final solution set is $f(x)=x^n$ for $n\ge 0$.
14.05.2023 20:01
Complete_quadrilateral wrote: Also could u please post your solution I would like to see it. Of course. Let $C=2023^{2023}+1$. The case when $P{}$ is constant is trivial, so we shall not discuss it. If $x\mid P(x)$ then write $P(x)=x^kQ(x)$ so that $x\nmid Q(x)$. Hence, when $\omega(n)\geqslant C$ we have \[\omega(n)\geqslant \omega(P(n))=\omega(n^kQ(n))\geqslant \omega(n),\]so we must have equality. That being said, if $\omega(n)\geqslant C$ all the prime factors of $Q(n)$ must divide $n{}$. Assuming that $Q(x)$ isn't the constant polynomial 1, we may take $n{}$ to be the product of $C$ large enough prime numbers, which do not divide $Q(0)$. Hence, $Q(n)>1$ is relatively prime to $n{}$, contradiction. So, in this case, we get the solution $P(x)=x^k$ with $k\geqslant 1$. If $x\nmid P(x)$ we take $n=\Pi\cdot q^k$ where $\Pi$ is the product of $C$ distinct prime numbers relatively prime to $P(0){}$ and $q$ is another prime number relatively prime to $P(0)$. It follows that $C+1\geqslant \omega(P(\Pi\cdot q^k))$ for all $k\geqslant 1$. To put it simply, $\omega(P(\Pi\cdot q^k))$ is bounded. Hence, we may take some $t\geqslant 1$ for which $\omega(P(\Pi\cdot q^t))$ is maximal and write \[P(\Pi\cdot q^t)=\prod_{i=1}^m{r_i}^{s_i}.\]Note that $q\neq r_i$ for all $i=1,\ldots,m$. Choose $M>\max(s_i)$ and for some large enough positive integer $B{}$ consider \[A=t+B\cdot\varphi\left(\prod_{i=1}^m {r_i}^M\right).\]Clearly, all of $r_1,\ldots,r_m$ divide $P(\Pi\cdot q^A)$. By the maximality of $\omega(P(\Pi\cdot q^t))$ it follows that only these prime numbers can divide $P(\Pi\cdot q^A)$. However, \[P(\Pi\cdot q^A)\equiv P(\Pi\cdot q^t)\bmod{r_i^M}\implies \nu_{r_i}(P(\Pi\cdot q^A))=s_i.\]Therefore, all of the exponents in the prime factorization of $P(\Pi\cdot q^A)$ are bounded: they are equal to $s_i$ for $r_i$ and 0 otherwise. Taking $B{}$ large enough makes $A{}$ large enough such that $|P(\Pi\cdot q^A)|$ tends to infinity, contradiction (recall that we assumed that $P{}$ is non-constant).
29.05.2023 07:37
Solved with hints from L567. Let $ P(x) = x^d \cdot Q(x)$ where $Q(0) \neq 0$. Assume possible $Q$ is non-constant. Call $k = 2023^{2023}$. By Schur we can pick pairwise distinct primes $p_1, p_2, \dots , p_{k+2} \in S$ such that for each $p_i$ we have a corresponding $b_i$; satisfying whenever $x \equiv b_i$ (mod $p_i$), we have $p_i \mid Q(x)$. Pick $p_i$ coprime to $Q(0)$ so that $p_i \nmid Q(0)$. Let $M = p_1p_2 \dots p_{k+2}$. By CRT, $x \equiv b_i$ (mod $p_i$) for all $i=1, 2, \dots, k+2$, is equivalent to $x \equiv c$ (mod $M$) for some $c$. Note $c$ is coprime with $M$. By Dirichlet, there is a prime $q$ such that $ q \equiv c$ (mod $M$). Choose $q_1, q_2, \dots, q_k$ to be any pairwise distinct primes, and different from all the primes picked till now. Consider $n = q (q_1 q_2 \dots q_k)^{\phi(M)}$. Note $\omega(n) = k+1$. But $n \equiv c$ (mod $M$). Hence $n \equiv b_i$ (mod $p_i$) and hence $p_i \mid Q(n) \mid P(n)$. Hence, $\omega(P(n)) \geqslant k+2$. But this contradicts given condition. Hence, $Q$ must be constant polynomial, say $c$. If $d \geqslant 1$: by $P(n)$ positive integer condition, $c \in \mathbb{N}$. If prime $p \mid c$, pick $n$ with $k+1$ prime factors none of which is $p$, to get contradiction. Hence $c=1$. This gives $P(x) = x^d$ as solution, which works. If $d=0$, $P \equiv c$, and it is easy to see precisely all $c$ such that $c \in \mathbb{N}$ and $\omega(c ) \leqslant k$ work. Hence, we are done!
28.01.2024 18:17
The only answer are $P(x)=c$ where $\omega(c)\le 2023^{2023}+1$ and $P(x)=x^n$ for positive integers $n$. It's easy to check that these polynomials satisfies the problem condition. Now, we prove that those are the only solutions. We're going to use this lemma: Lemma 1. For any nonconstant polynomial P with integer coefficients, the set of all primes dividing $P$ is infinite. Proof. It's well known that there's infinitely many primes. Assume otherwise, let $p_1,p_2,...,p_N$ be the complete set of primes dividing $P(x)=a_nx^n+a_{n-1}x^{n-1}+...+a_1x+a_0$. If $a_0=0$, we can $x$ as the product of $N+1$ primes to get a contradiction. If $a_0\neq 0$. Notice that $P(a_0x)=a_na_0^nx^n+a_{n-1}a_0^{n-1}x^{n-1}+...+a_1a_0x+a_0=a_0\left( a_na_0^{n-1}x^n+a_{n-1}a_0^{n-2}x^{n-1}+...+a_1x+1 \right)$. We take $x=p_1p_2...p_N$. Now, $P(a_0p_1p_2...p_N)$ must have new prime divisor, a contradiction. $\blacksquare$ Now, we let $L=2023^{2023}$. It's clear that the first coefficient of $P$ must be a positive integer. Note that $P(x)=c$ where $\omega(c)\le L+1$ always satisfies the problem condition. Now, assume that $P$ is non constant. We write $P(x)=a_nx^n+a_{n-1}x^{n-1}+...+a_1x+a_0$. Case 1. If $a_0=0$, then let $P(x)=x^kQ(x)$ where $k \ge 1$, and the last coefficient in $Q(x)$ is nonzero. (a) If $Q$ is constant, we must have $|Q(x)|=1$, which forces $P(x)=x^n$. Otherwise, we can select $n$ to be the product of any $L+1$ primes that are greater than $Q(0)$ to get a contradiction. (b) If $Q$ is not constant, we select $n$ to be the product of $L+1$ primes that are greater than $Q(0)$ to get a contradiction. Case 2. If $a_0\neq 0$, let $p_1,p_2,...$ be the prime divisors of $P$ that are relatively prime to $a_0$ (there are still infinitely many of them). Let $p_i\mid P(x_i)$ for all $i=1,2,...,L+1$. We now choose $x\equiv x_i \pmod{p_i}$ for all $i=1,2,...,L+2$ (this exists by CRT) to get $p_1p_2...p_{L+2}\mid P(x)$. Let the CRT implies $x\equiv S \pmod{p_1p_2...p_{L+2}}$. We now have $\gcd(S,p_i)=1$ for all $i=1,2,...,L+2$. Now we choose primes $q_1,q_2,...,q_{L}\equiv 1\pmod{p_1p_2...p_{L}}$ and $q_{L+1}\equiv S \pmod{p_1p_2...p_{L+2}}$ (it exists by Dirichlet's Theorem). We now have $q_1q_2...q_{L+1}\equiv S \pmod{p_1p_2...p_{L+2}}$. It means that the number $$A=\frac{q_1q_2...q_{L+1}-S}{p_1p_2...p_{L+2}}$$is an integer. Now, we take $n=Ap_1p_2...p_{L+2}+S=q_1q_2...q_{L+1}$. Clearly, $\omega(n)=L+1$, but $\omega(P(n))\ge L+2$ which gives us a contradiction. Hence, the only answer are $P(x)=c$ where $\omega(c)\le 2023^{2023}+1$ and $P(x)=x^n$ for positive integers $n$, and it's easy to check that these polynomials satisfies the problem condition.
05.08.2024 20:54
I really enjoyed this problem and it was quite simple for P3. My solution is basically the same as the ones above.
03.09.2024 17:16
This is similar to the others. The only polynomials satisfy the conditions are $P\equiv c$ where $\omega(c)\leq 2023^{2023}+1$ and $P(n)=n^k$ where $k\in Z^+$. Suppose that $P$ is non-constant. Case $1$: The constant term of $P$ is not $0$. Let $c$ be the constant term. Rewrite $P(n)=nQ(n)+c$. If $Q\equiv 0,$ then $P\equiv 0$ holds. Suppose that $Q\not \equiv 0$. By Schur's theorem, we can choose $q_i|P(r_i)$ for $l$ primes. Take $q\equiv r_i(mod \ q_i)$ which is possible by Drichlet and chinese remainder theorem. We have $q_1...q_l|P(n)$ since $q_i|P(q)-P(r_i)$. Take a prime $p\equiv 1(mod \ q_1...q_l)$. We see that $ptQ(pt)\equiv tQ(pt)\equiv tQ(t)\equiv -c(mod \ q_1...q_l)$. Thus, $q_1...q_l|ptQ(pt)+c$. We can increase the number of distinct prime divisors of $t$. So we can find $q_1...q_l|nQ(n)+c$ for any $l,\omega(n)$. Note that $\omega(t)$ is equal to $1$ in the beggining and it is increased by exactly one at each step. Choose $l>\omega(n)>2023^{2023}$ which gives a contradiction. Case $2$: The constant term of $P$ is $0$. Let $P(n)=n^kR(n)$ where $R(0)\neq 0$. We have $\omega(n)\geq \omega(P(n))=\omega(n^kR(n))\geq \omega(R(n))$. The constant term of $R$ is not $0$ so $R$ must be constant. Let $R\equiv r$. $P(n)=rn^k$ and $\omega(n)\geq \omega(rn^k)$. If $r\neq 1,$ then we can choose $(n,r)=1$ and in this case $\omega(n^kr)=\omega(n)+\omega(r)\geq \omega(n)+1>\omega(n)$ which results in a contradiction. Thus, $r=1$ and $P(n)=n^k$ for any $k$ positive integer.$\blacksquare$
13.01.2025 14:31
This problem was magical: Let $C=2023^{2023}$. If $P$ is a constant, then we are done. Assume that: $P$ is non-constant. If $P(0)=0$, then let $P(x) = x^k Q(x)$ where $Q(0) \neq 0$. Then, if $Q$ is constant, then let $Q \equiv c$. Plugging $n=p_1 p_2 \cdots p_{C+1}$ such that some prime $p|C$ and $p \neq p_i$ for all i. Then: $\omega(n) = C+1$ but $\omega(P(n)) \ge C+2$. Therefore, contradiction. If $Q$ is non-constant, then: by Schur's theorem, we have: $Q$ to have infinitely many prime divisors. Let $q_1, \cdots, q_T$ denote such divisors where $T$ is sufficiently larger than $C$, with $p_i | Q(x_i)$ for some $x_i \in \mathbb Z$. Construct primes distinct primes $p_1, \cdots, p_{C+1}$: $p_1 \equiv p_2 \cdots \equiv p_C \equiv 1 \pmod q_i$ and $p_{C+1} \equiv x_i \pmod q_i$. We can construct such primes due to CRT and Dirichlet's theorem (on Arithmetic Progressions). Taking $n=p_1 p_2 \cdots p_{C+1}$ gives: $\omega(n)=C+1$ but $\omega(P(n)) \ge T >> C$ as $P(n) \equiv P(x_i) \equiv 0 \pmod q_i$ for all $1 \le i \le T$. Therefore, contradiction and we are done. Therefore, the only solutions are: $P \equiv t$ for some constant $t$ with $\omega(t) \le C$ and $P(x) = x^k$ for some $k \in \mathbb N$.