$p$ is a prime number such that its remainder divided by 8 is 3. Find all pairs of rational numbers $(x,y)$ that satisfy the following equation.
$$p^2 x^4-6px^2+1=y^2$$
Hint: just observe that: ${{p}^{2}}{{x}^{4}}-6p{{x}^{2}}+1={{y}^{2}}\Leftrightarrow {{(p{{x}^{2}}-3)}^{2}}-8={{y}^{2}}\Leftrightarrow (p{{x}^{2}}-y-3)(p{{x}^{2}}+y-3)=8$ and now casework
gordian.knot wrote:
Hint: just observe that: ${{p}^{2}}{{x}^{4}}-6p{{x}^{2}}+1={{y}^{2}}\Leftrightarrow {{(p{{x}^{2}}-3)}^{2}}-8={{y}^{2}}\Leftrightarrow (p{{x}^{2}}-y-3)(p{{x}^{2}}+y-3)=8$ and now casework
It's not done
Claim) If integers $a,b,c$ satisfy $p^2 a^4 -6pa^2b^2+b^4=c^2$, then $abc=0$
Suppose there exists $a,b,c$ that satisfy the condition and $abc\neq 0$.
We can assume $a,b,c$ are natural number.
Let's get a pair of $(a,b,c)$ which $b$ is the smallest.
Let) $d=pa^2-3b^2$
$d^2-8b^4=c^2$
$8b^4=(d+c)(d-c) \cdots $(1)
Let) $x=gcd(c,d), e=\frac{c}{x}, f=\frac{d}{x}$
$(gcd(e,f)=1)$
by (1) $x|b $
Let) $g=\frac{b}{x}$
$8x^2g^4=(f+e)(f-e)$
$2|f+e, 2|f-e$
$2x^2g^4=\frac{f+e}{2} \frac{f-e}{2}$
as $gcd(\frac{f+e}{2}, \frac{f-e}{2})=1, \{\frac{f+e}{2}, \frac{f-e}{2}\}=\{2h^2, j^2\}$ for some natural numbers $h,j$
we can know $gcd(h,j)=1, 2\nmid j $
$f=2h^2+j^2, hj=xg^2$
by definition of $d$,
$pa^2=d+3b^2=xf+3x^2g^2=x(2h^2+j^2+3hj)$
$pa^2g^2=hj(2h+j)(h+j) \cdots$ (2)
as$ j$ is odd number, $gcd(j, 2h+j)=1 $
therefore $h,j,h+j,2h+j$ is pairwise coprime
we can know that one of them is a form of $p*(square)$, while other are $square$
now casework
I) $h+j$ is $p*(square)$
$(square)+(square)=h+j=h+j=p*(square)$
by modulo 4, we can get $h,j,h+j$ has to be even number. contradiction.
II) $2h+j $ is $p*(square)$
$(square)+(square)=(h+j)+h=2h+j=p*(square)$
likewise I), contradiction.
III) $j$ is $p*(square)$
as $j$ is odd number, $j\equiv 3 (mod 8)$
$h+3\equiv h+j (mod 8)$, so $h\equiv 1 (mod 8), h+j\equiv 4 (mod 8)$
$2h+j\equiv 5(mod 8)$, so contradiction.
IV) $h$ is $p*(square)$
by similar modulo work as III), we can get $h$ is even number where $j,h+j,2h+j$ is odd number.
Let) $h=4pk^2, j=\gamma ^2, h+j=m^2, 2h+j=n^2$
as $m,n$ is odd number, $\frac{n-m}{2} \frac{n+m}{2}=\frac{-(h+j)+(2h+j)}{4}=\frac{h}{4}=pk^2 \cdots (2)$
as $gcd(m,n)=1$, $\{\frac{n-m}{2}, \frac{n+m}{2}\}=\{p\alpha^2, \beta^2\}$ for some natural numbers $\alpha, \beta$.
therefore $n=|p\alpha^2-\beta^2|, m=p\alpha^2+\beta^2$
by (2) $k=\alpha \beta$
therfore $\gamma^2=j=h+j-h=n^2-4pk^2=(p\alpha^2-\beta^2)^2-4p\alpha^2 \beta^2=p^2\alpha^4-6p\alpha^2\beta^2+\beta^4 \cdots$ (3)
$b=xg\ge\sqrt{xg^2}=\sqrt{hj}\ge \sqrt{h}=\sqrt{4p\alpha^2\beta^2}\ge \beta \cdots$ (4)
by (3), (4), contradiction
Therefore contradiction on all cases, so Claim is right.
chrono223 wrote:
Claim) If integers $a,b,c$ satisfy $p^2 a^4 -6pa^2b^2+b^4=c^2$, then $abc=0$
Suppose there exists $a,b,c$ that satisfy the condition and $abc\neq 0$.
We can assume $a,b,c$ are natural number.
Let's get a pair of $(a,b,c)$ which $b$ is the smallest.
Let) $d=pa^2-3b^2$
$d^2-8b^4=c^2$
$8b^4=(d+c)(d-c) \cdots $(1)
Let) $x=gcd(c,d), e=\frac{c}{x}, f=\frac{d}{x}$
$(gcd(e,f)=1)$
by (1) $x|b $
Let) $g=\frac{b}{x}$
$8x^2g^4=(f+e)(f-e)$
$2|f+e, 2|f-e$
$2x^2g^4=\frac{f+e}{2} \frac{f-e}{2}$
as $gcd(\frac{f+e}{2}, \frac{f-e}{2})=1, \{\frac{f+e}{2}, \frac{f-e}{2}\}=\{2h^2, j^2\}$ for some natural numbers $h,j$
we can know $gcd(h,j)=1, 2\nmid j $
$f=2h^2+j^2, hj=xg^2$
by definition of $d$,
$pa^2=d+3b^2=xf+3x^2g^2=x(2h^2+j^2+3hj)$
$pa^2g^2=hj(2h+j)(h+j) \cdots$ (2)
as$ j$ is odd number, $gcd(j, 2h+j)=1 $
therefore $h,j,h+j,2h+j$ is pairwise coprime
we can know that one of them is a form of $p*(square)$, while other are $square$
now casework
I) $h+j$ is $p*(square)$
$(square)+(square)=h+j=h+j=p*(square)$
by modulo 4, we can get $h,j,h+j$ has to be even number. contradiction.
II) $2h+j $ is $p*(square)$
$(square)+(square)=(h+j)+h=2h+j=p*(square)$
likewise I), contradiction.
III) $j$ is $p*(square)$
as $j$ is odd number, $j\equiv 3 (mod 8)$
$h+3\equiv h+j (mod 8)$, so $h\equiv 1 (mod 8), h+j\equiv 4 (mod 8)$
$2h+j\equiv 5(mod 8)$, so contradiction.
IV) $h$ is $p*(square)$
by similar modulo work as III), we can get $h$ is even number where $j,h+j,2h+j$ is odd number.
Let) $h=4pk^2, j=\gamma ^2, h+j=m^2, 2h+j=n^2$
as $m,n$ is odd number, $\frac{n-m}{2} \frac{n+m}{2}=\frac{-(h+j)+(2h+j)}{4}=\frac{h}{4}=pk^2 \cdots (2)$
as $gcd(m,n)=1$, $\{\frac{n-m}{2}, \frac{n+m}{2}\}=\{p\alpha^2, \beta^2\}$ for some natural numbers $\alpha, \beta$.
therefore $n=|p\alpha^2-\beta^2|, m=p\alpha^2+\beta^2$
by (2) $k=\alpha \beta$
therfore $\gamma^2=j=h+j-h=n^2-4pk^2=(p\alpha^2-\beta^2)^2-4p\alpha^2 \beta^2=p^2\alpha^4-6p\alpha^2\beta^2+\beta^4 \cdots$ (3)
$b=xg\ge\sqrt{xg^2}=\sqrt{hj}\ge \sqrt{h}=\sqrt{4p\alpha^2\beta^2}\ge \beta \cdots$ (4)
by (3), (4), contradiction
Therefore contradiction on all cases, so Claim is right.
by Claim, solutions are $(0,\pm 1)$.
Wrong. $8b^4=d^2-c^2$ doesn't mean that $gcd(c,d)|b$, it just means that $gcd(c,d)|b^2$.