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Thursday, December 4, 2014

Gazeta Matematica 4/2014, Problem E:14641

Problem:
Find all primes p such that the equation x^3+y^3+3xy=p^2+1 has integer solutions.

Proposed by Alessandro Ventullo

Solution:
Observe that x^3+y^3-1+3xy=(x+y-1)(x^2+y^2+1-xy+x+y). So, the given equation can be rewritten as
(x+y-1)(x^2+y^2+1-xy+x+y)=p^2. Since x^2+y^2+1-xy+x+y=\dfrac{1}{2}[(x-y)^2+(x+1)^2+(y+1)^2] \geq 0, then x+y-1\geq0, i.e. x+y \geq 1. This gives x^2+y^2+1-xy+x+y\geq 2|xy|+1-xy+1=2|xy|-xy+2 \geq 2. Moreover, x^2+y^2+1-xy+x+y>x+y-1. So, it must be
\begin{array}{rll} x+y-1&=&1 \\ x^2+y^2+1-xy+x+y&=&p^2. \end{array}
The first equation gives

x+y=2                                (1)

the second equation gives (x+y)^2-3xy+(x+y)+1=p^2, i.e.

xy=\dfrac{7-p^2}{3}.                                (2)

So, t^2=\left(\dfrac{x-y}{2}\right)^2=\dfrac{(x+y)^2-4xy}{4}=1-\dfrac{7-p^2}{3}, i.e. 3t^2=p^2-4. If p>2, we have that 3t^2 \equiv 0,3 \pmod{4} and p^2-4 \equiv 1 \pmod{4}, a contradiction. So, p=2. This value is acceptable because, from equations (1) and (2), we get (x,y)=(1,1).

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