Prove that $4x^3-3x+1=2y^2$ has at least $31$ solutions in positive integers $x,y$ with $x\le 1980$. Variant: Prove the equation $4x^3-3x+1=2y^2$ has infinitely many solutions in positive integers x,y.
Problem
Source: IMO 1980 Great Britain #1
Tags: algebra unsolved, algebra
GlassBead
14.06.2011 18:12
Multiplying both sides by two, we have $8x^3-6x+2=4y^2=(2x)^3-3(2x)+2=(2x-1)^2(2x+2)=(2y)^2$. For this to have integer solutions, we must have that $2x+2$ be a perfect square.
The least positive integer solution for $x$ is $x=1$, where $2x+2$ is a square. We can see that each $x$ (let us denote the $n$-th integer solution of $x$ as $a_n$) can be expressed as $a_n=2n^2-1, a_1=1$. Since $x \leq 1980$, $2n^2-1 \leq 1980$. Solving for the maximum of $n$, we have that $\max n \approx 31.4722$, which means that there are at least $31$ possible integer values of $x$ that yield an integer $y$.
To prove the variant, we note that $2n^2-1$ is monotonically increasing for positive $n$, thus there are an infinite number of integer $x$ that yield an integer $y$.
conejita
14.06.2011 18:13
$2y^2=4x^3-3x+1=(x+1)(2x-1)^2$ After that, we can note that the pair $(x,y)=(2t^2-1,4t^3-3t)$ is a solution. Now, because $x\leq1980$ is a positive integer, this mean, $0<t^2\leq990$ Then $t=1,2,3,...,31$. In conclusion, the original equation has at least $31$ solutions with the conditions.