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Prime numbers, diophantine equations, diophantine approximations, analytic or algebraic number theory, arithmetic geometry, Galois theory, transcendental number theory, continued fractions

7 votes
Accepted

Generalizing Ramanujan's "1729 story"

There are many articles that study the quantity you call $r_n(k)$ using sieve methods. Among them I mention the following, which give highly non-trivial bounds for the number of $k<X$ such that $r_n(k …
Joe Silverman's user avatar
4 votes

sum of four squares with some coefficients

First, one doesn't normally call $(a,b,c,d)$ a "pair" of rational numbers. In any case, first check that your equation has a solution with $(b,d)\ne0$ in $\mathbb R$ (which it clearly does) and in $\m …
Joe Silverman's user avatar
5 votes

Have you ever seen this product?

Elementary comment. For a fixed real number $x$, define a multiplicative function $f(x;n)$ as follows. Factor $n=p_1^{e_1}\cdots p_r^{e_r}$ and set $f(x;n)=x^{e_1+\cdots+e_r}$. Then $$ \prod \left(1 …
Joe Silverman's user avatar
3 votes

Divisibility among discriminants

There are a number of papers that deal with fields generated by points in the inverse image of iterates. So in your setting, let $c=f(f(r))=f^2(r)$, then $f(r)$ is in $f^{-1}(c)$, the first inverse im …
Joe Silverman's user avatar
10 votes

Quadratic squares

A non-trivial answer for your comment/question for an example giving $\Omega(\log N)$, take $p(x)=2x^2+1$. This gives a Pell equation $2x^2+1=y^2$, and taking powers of the fundamental unit will, I be …
Joe Silverman's user avatar
10 votes

Effective Mordell

If one had an effective version of the abc conjecture, then Elkies showed how to use it to obtain an effective version of the Mordell conjecture (using Belyi maps). In your formulation, the theorem w …
Joe Silverman's user avatar
7 votes
Accepted

Given any $(a,m,n)\in \mathbb{Z}\times\mathbb{N}^2$ with $\gcd(a,m)=1$ is there a quick way ...

The Goldwasser-Micali probabilistic cryptosystem is based on exactly this principle. Let $N=pq$ and let $a$ be an integer with $\left(\frac{a}{p}\right)=\left(\frac{a}{q}\right)=-1$, i.e., $a$ is a no …
Joe Silverman's user avatar
3 votes
Accepted

Bounded differences in exponential sequences

For any fixed integers $a$ and $b$ that are multiplicatively independent and for any integer $k$, there are only finitely many pairs of positive integers $m$ and $n$ such that $$ a^n - b^m = k. $$ To …
Joe Silverman's user avatar
4 votes

Computing a polynomial product over roots of unity

The transformation $$ \mathcal{L}_n : \mathbb{C}(x) \longrightarrow \mathbb{C}(x) $$ defined by $$ \mathcal{L}_n(F(x)) = \sum_{\zeta\in\boldsymbol\mu_n} F(\zeta y)\Big|_{y^n\to x} $$ is called a …
Joe Silverman's user avatar
5 votes

Conjecture on Markov-Hurwitz Diophantine equation

Not an answer, but a bit long for a comment. If you don't already know them, you might find the papers of Baragar to be of interest: [1] The exponent for the Markoff-Hurwitz equations. Pacific J. Mat …
Joe Silverman's user avatar
9 votes
Accepted

Local-to-global principle for certain genus 0 curves

The former always has rational solutions. Let $x=tu$, $y=tv$, $z=tw$, then $$ au^2 + bv^2 = c t w^3. $$ So simply choose any $u,v,w\in\mathbb Q^*$ that you want, set $$ t = \frac{au^2+bv^2}{cw^3}, $$ …
Joe Silverman's user avatar
3 votes

A question on degree 4 binary forms

Have you left some condition off of Stewart's conjecture? It's not right as you've stated it. Indeed: Theorem Let $f(x,y)\in\mathbb{Z}[x,y]$ be a polynomial of degree $3$ with $\operatorname{Disc}(f …
Joe Silverman's user avatar
8 votes
Accepted

Sequences satisfying gcd(S(x), S(y)) = S(gcd(x,y))

A strong divisibility sequence is a sequence of positive integers $(a_n)_{n\ge1}$ with the property that $\gcd(a_n,a_m)=a_{\gcd(n,m)}$. See http://en.wikipedia.org/wiki/Divisibility_sequence. (Strong) …
Joe Silverman's user avatar
5 votes

When are infinitely many points in the orbit of a polynomial integers?

A slight variant that turns out to be non-elemenatry is to replace the polynomial with a rational function. This leads to: Theorem: Let $R(x)\in\mathbf{Q}(x)$ be a rational function of degree at leas …
Joe Silverman's user avatar
5 votes

Intersection of $\{2^a 3^b 5^c 7^d\}$ and its translates

As Mike Bennett said, this is an example of an $S$-unit equation, although you are asking for solution to $u-v=1$ in $\mathbb Z_S^*\cap \mathbb Z$. More generally, one drops the requirement that $u$ a …
Joe Silverman's user avatar

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