# Polynomial representation of sequences of length $n$ taken modulo $m$

By Newton interpolation (or other standard methods), one can easily determine the number and structure (the latter to a lesser extent) of sequences $(a_1,a_2,\ldots,a_n)\pmod{n}$ represented by integer polynomials given the prime factorization of $n$ (note that we can reduce to the prime power case by the Chinese remainder theorem). For instance, see this AoPS thread. However, for such $f\in\mathbb{Z}[x]$ we have strong restrictions like $u-v\mid f(u)-f(v)$ for integers $u,v$.

IMO it's then natural to wonder about (e.g. the new structure of valid $f$) for rational polynomials in general, where standard interpolation methods (in certain mods) aren't as clean. (Mostly copied from my comment below.)

To this end, consider a pair of positive integers $(n,m)$ with $n,m>1$ such that for every sequence $(a_1,a_2,\ldots,a_n)\in(\mathbb{Z}/m\mathbb{Z})^n$, there exists an integer-valued polynomial $f\in\mathbb{Q}[x]$ satisifying $f(x)\equiv a_i\pmod{m}$ whenever $x\equiv i\pmod{n}$.

First, it is easy to show that $n,m$ must be powers of the same prime. Indeed, if there exist distinct primes $p,q$ such that $p\mid n$ and $q\mid m$, then for some sufficiently large integer $\ell$, we have $q\mid f(x+n)-f(x)$ and $q\mid f(x+q^\ell)-f(x)$ for every $x$. But $\gcd(n,q^\ell)\mid n/p$, so by Bézout's identity, $q\mid f(x+n/p)-f(x)$ for all $x$ and thus the $a_{i(n/p)}$ must all be congruent $\pmod{q}$ in order for $f$ to exist.

On the other hand, if $n=p^i$ and $m=p^j$ for some prime $p$ and positive integers $i,j$, then given a sequence $(a_1,a_2,\ldots,a_n)$, consider the polynomial $g(x)=\sum_{k=1}^{n}a_kx^k$. By a simple induction on $d\ge0$, we can show (using finite differences) that a degree $d$ polynomial $f$ (for convenience, say $\deg{0}=-1$) satisfying the desired properties exists iff $d$ is the smallest number such that the division of $(x-1)^{d+1}g(x)$ by $x^n-1$ gives a remainder $r(x)$ with coefficients all divisible by $m$ (call $d+1$ the order of the sequence $(a_1,a_2,\ldots,a_n)$, so the all-zero sequence has order $0$). Since $(x-1)^n=x^n-1$ in $F_p$, we have $(x-1)^{jn}=(x^n-1)u(x)+p^j v(x)$ for some polynomials $u,v$ with integer coefficients, so the order of any sequence is finite and at most $jn$.

However, this leads to the following two questions:

1. For fixed $p,i,j$, what is the maximum possible order $M$ of a sequence $(a_1,a_2,\ldots,a_n)\in(\mathbb{Z}/m\mathbb{Z})^n$? (Resolved in the update.)
2. How many sequences are there of order $r$, where $r\in[0,M]$ is a fixed integer?
3. Is there a reasonably nice way to describe the sequences of a fixed order $r$ (possibly in terms of one of the corresponding polynomials $f,g$)?

Looking at small cases, it seems that the answer to 1 should be $p^i+(j-1)\phi(p^i)$, where $\phi$ denotes Euler's totient function.

Update: OK, I think I have a (messy) proof that the answer to question 1 is indeed $p^i+(j-1)\phi(p^i)$, but it doesn't seem to lend itself to 2 or 3 in any way.

Fix $p,i$. First note that since $(x-1)^{p^k}=x^{p^k}-1$ (in $F_p$) for $k=i$ and $k=i-1$, we have $(x-1)^{\phi(p^i)} = \Phi_{p^i}(x)$ in $F_p$ as well, so $(x-1)^{\phi(p^i)}=\Phi_{p^i}(x)+p T(x)$ for some integer polynomial $T$ of degree at most $\phi(p^i)-1$, where $\Phi_t$ denotes the $t^{th}$ cyclotomic polynomial. Observe that $T(1)=-1$, so $1$ is not a root of $T$ in $F_p$.

Using this key fact, we will induct on $j\ge1$ to construct a sequence of integer polynomials $P_j,Q_j$ such that $$(x-1)^{p^i+(j-1)\phi(p^i)} = (x^{p^i}-1)P_j(x)+p^j(x-1)Q_j(x),$$ $v_p(Q_j(1))=i-1$, and in $F_p$, $(x-1)^{p^{i-1}-1}\| Q_j(x)$ (i.e. $1$ is a root of multiplicity $p^{i-1}-1$).

For $j=1$, we simply take $P_1(x)=1$ and $Q_1(x) = \frac{(x-1)^{p^i}-(x^{p^i}-1)}{p(x-1)}$, where clearly $Q_1(1)=-p^{i-1}\implies v_p(Q_1(1))=i-1$. Showing $(x-1)^{p^{i-1}-1}\| Q_j(x)$ is slightly harder, but not too bad. It's easy to show by counting prime factors that $\binom{p^i}{k}$ is divisible by $p$ for all $1\le k\le p^i-1$ and not divisible by $p^2$ iff $p^{i-1}\mid k$. Furthermore, by Babbage's theorem we have $\binom{p^i}{kp^{i-1}}\equiv\binom{p}{k}\pmod{p^2}$ for $1\le k\le p-1$. Hence for $p=2$, we just need to show that $(x-1)^{p^{i-1}}\| x^{p^{i-1}}-1$ in $F_2$, which is obvious; for $p>2$ odd, we need to show $$(x-1)^{p^{i-1}} \| \sum_{k=1}^{p-1}\frac{x^{kp^{i-1}}}{k} = \left(\sum_{k=1}^{p-1}\frac{x^k}{k}\right)^{p^{i-1}}$$ in $F_p$ (note that $k^p=k$ by Fermat's little theorem). But if $h(x)=\sum_{k=1}^{p-1}\frac{x^k}{k}$, then $h(1)\equiv0\pmod{p}$ while $h'(1)\equiv p-1\pmod{p}$, so $1$ is a simple root of $h$ and we're done with the base case.

Now assuming the result for some $j\ge1$ (so that $\frac{x^{p^{i-1}}-1}{x-1}\mid Q_j(x)$ in $F_p$), we can write $Q_j(x)=\frac{x^{p^{i-1}}-1}{x-1}R(x)+pS(x)$ for two integer polynomials $R,S$ with $\deg{S} < p^{i-1}-1$. (+) Then $$(x-1)^{p^i+j\phi(p^i)} = (x-1)^{p^i+(j-1)\phi(p^i)} (x-1)^{\phi(p^i)}$$ can be written as $$(x^{p^i}-1)P_j(x)(x-1)^{\phi(p^i)}+p^{j+1}(x-1)T(x)Q_j(x)+p^j\Phi_{p^i}(x)(x-1)Q_j(x)$$ or equivalently after substitution, $$(x^{p^i}-1)(P_j(x)(x-1)^{\phi(p^i)}+p^j R(x))+p^{j+1}(S(x)\Phi_{p^i}(x)+T(x)Q_j(x)),$$ so we can take $$P_{j+1}(x) = P_j(x)(x-1)^{\phi(p^i)}+p^j R(x)$$ and $$Q_{j+1}(x) = S(x)\Phi_{p^i}(x)+T(x)Q_j(x).$$ As $$(x-1)^{p^{i-1}}\mid (x-1)^{\phi(p^i)}=\Phi_{p^i}(x)$$ in $F_p$ and $T(1)=-1$, we see that $(x-1)^{p^{i-1}-1}\| Q_{j+1}(x)$.

It remains to show that $v_p(Q_{j+1}(1))=i-1$. By (+) and the definition of $Q_{j+1}$, we find $Q_{j+1}(1)=\Phi_{p^i}(1)S(1)+T(1)Q_j(1)=pS(1)-Q_j(1)=-p^{i-1}R(1)$, so $v_p(Q_{j+1}(1))\ge i-1$. However, if $p^i\mid Q_{j+1}(1)$, then $p\mid R(1)$, so writing (+) in $F_p$ we have $Q_j(x) = (x-1)^{p^{i-1}-1}R(x)$. But then $(x-1)^{p^{i-1}}\mid Q_j(x)$, contradicting our inductive hypothesis.

Thus our induction is complete.

Clearly this construction shows that the order of any sequence is at most $M=p^i+(j-1)\phi(p^i)$. On the other hand, it is easy to show that the order of $(1,0,\ldots,0)$ is $M$. Indeed, note that $g(x)=x$ for this sequence, and suppose $x(x-1)^{M-1}/(x^{p^i}-1)$ leaves a remainder with coefficients all divisible by $p^j$. From the induction statement, we have $$(x-1)^{M-1} = \frac{x^{p^i}-1}{x-1}P_j(x)+p^jQ_j(x),$$ so writing $P_j(x)=(x-1)U(x)+V$ for an integer $V$, we get $p^j\mid V$. But then plugging in $1$ to this equation, $0=(0)U(1)+(p^i)V+p^j Q_j(1)$, whence $p^i\mid Q_j(1)$, contradiction.

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I'm curious what's the motivation for this question? –  tweetie-bird Jul 15 '12 at 15:33
By Newton interpolation, one can easily determine the number and structure (the latter to a lesser extent) of sequences $(a_1,a_2,\ldots,a_n)\pmod{n}$ represented by integer polynomials nicely given the prime factorization of $n$ (esp. for prime powers). However, for such $f$ we have restrictions like $u-v\mid f(u)-f(v)$ for integers $u,v$. IMO it's then natural to wonder about (e.g. the new structure of valid $f$) for rational polynomials in general. Standard interpolation methods aren't as clean, and I don't see a clear connection between sequences and their polynomial representations. –  Victor Wang Jul 15 '12 at 16:47
Thanks! That makes sense! –  tweetie-bird Jul 15 '12 at 20:37
@Victor: just a word of warning (since you mention being new), if you edit your post 8 times then it becomes community wiki. In that case, people who answer your question don't get reputation points for answering. I'm afraid I probably can't help you with answering your question, but just thought I'd let you know how this works. –  tweetie-bird Jul 16 '12 at 0:11
Oh, I didn't know that. Thanks! –  Victor Wang Jul 16 '12 at 0:37