TITLE: Does the elliptic curve $y^2 = 4 x^3 -6075$ have any integer points?
QUESTION [7 upvotes]: Let $E$ be the elliptic curve $y^2 = 4 x^3 -6075$. I ran the following  Mathematica code, which searches naively for integer solutions to $E$ but it did not find any solutions $(x, y) \in E(\mathbb{Z})$ satisfying $0 \leq x \leq 10^6$. 
T = Table[ z = 4 x^3 - 6075; 
      If[ IntegerQ[Sqrt[z]], 
         {x, Sqrt[z]}
      , 0]
  , {x, 0, 1000000}];
DeleteCases[T, 0] 

Is $E(\mathbb{Z})$ empty? Also, what is the easiest database on Mordell curves to access? I have had some trouble installing the Pari database in the past.

REPLY [5 votes]: No, $y^2=4x^3-6075$ has no integer solution. 
An elementary observation: $3\nmid x$: because $$3\mid x \implies 27\mid y^2 \implies 9\mid y \implies 
9\mid x$$ but $y^2+6075$ has no solution in $\mathbb{Z}/3^6\mathbb{Z}$. 
The ring of integer of $\mathbb{Q}(\sqrt{-3})$ is a UFD. The equation can be written as
$$\left( {\frac{{y + 45\sqrt { - 3} }}{2}} \right)\left( {\frac{{y - 45\sqrt { - 3} }}{2}} \right) = {x^3}$$
elements inside brackets, denoted by $\alpha$ and $\beta$, are integral over $\mathbb{Z}$ since $y$ is odd. 
I claim that $\alpha,\beta$ are relatively prime. If a prime $\pi$ divides both, then $\pi\mid 45\sqrt{-3}$, so $\pi = \sqrt{-3}$ or $5$. If $\pi = \sqrt{-3}$, then $3\mid x$, contradiction. If $\pi = 5$, 
let $v_5$ denote valuation at $5$, normalized so that $v_5(5)=1$, note that $v_5(\alpha) \in \mathbb{Z}$ as $v_5$ is unramified. $$0< v_5(\alpha)+v_5(\beta) = 2v_5(\alpha)= 3v_5(x) $$
this says $v_5(x)$ is even, hence $5^6 \mid (y^2+6075)$, but $y^2+6075=0$ has no solution in $\mathbb{Z}/5^6\mathbb{Z}$, ruling out $\pi = 5$. Valuation can be saved by noting that $5^3 \mid (y^2+6075)$ is already impossible, but it seems difficult to deduce the stronger $5^6 \mid (y^2+6075)$ from consideration in $\mathbb{Z}$ alone.
Since $\alpha,\beta$ are relatively prime, they are both cube, say
$$\frac{{y + 45\sqrt { - 3} }}{2} = {(\frac{{a + b\sqrt { - 3} }}{2})^3} \qquad \text{ or } \qquad \left( {\frac{{1 + \sqrt { - 3} }}{2}} \right){\left( {\frac{{a + b\sqrt { - 3} }}{2}} \right)^3}$$
with $a,b$ both even or odd. The first case gives $60= {a^2}b - {b^3}$, so $b$ has only a few possible values, checking them gives no integer solutions to original equation. The second case gives $$\frac{{45}}{2} = \frac{{{a^3} + 3{a^2}b - 9a{b^2} - 3{b^3}}}{{16}}$$ this says $v_3(a) \geq 1$, thus $v_3(a^3+3a^2b-9ab^2)\geq 3$ but $v_3(45/2) = 2$, so $v_3(3b^3) = 2$, absurd.