Four variables and one equation?

How many ordered quadruples ( x , y , z , p ) (x, y, z, p) satisfy x 3 + y 3 p + z 3 p 2 = 0 x^{3}+y^{3}p+z^{3}p^{2}=0 where x x , y y , z z are non-zero integers and p p is a prime?


The answer is 0.

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2 solutions

Abhishek Sinha
Mar 19, 2014

Let x , y , z , p x,y,z,p be a solution of the above diophantine equation. We can write x x as x = ± p m r x=\pm p^m r , where m 0 m \geq 0 is the highest power of p p that divides x x (This can be zero, if x x is not divisible by p p ). From the given equation, it is easy to see that p x 3 p|x^3 , i.e. p x p|x . So, let x = p x 1 x=px_1 , where x 1 x_1 is an integer. Then we have the following equation

p 3 x 1 3 + y 3 p + z 3 p 2 = 0 p^3x_1^3+y^3p+z^3p^2=0 ,

i.e. y 3 + z 3 p + x 1 3 p 2 = 0 y^3+z^3p+x_1^3p^2=0

This implies that p y p|y , So let y = p y 1 y=py_1 , where y 1 y_1 is an integer. We have the similar equation again z 3 + x 1 3 p + y 1 3 p 2 = 0 z^3+x_1^3p+y_1^3p^2=0 . Thus, p z p|z . Let z = p z 1 z=pz_1 , with z 1 z_1 being an integer. Hence we have,

x 1 3 + y 1 3 p + z 1 3 p 2 = 0 x_1^3+y_1^3p+z_1^3p^2=0

Which has the exact same form of the diphantine equation we started with. Hence we can repeat this step more than m + 1 m+1 times, to conclude that p m + 1 x p^{m+1}|x , which contradicts the fact that m m is the highest power of p p that divides x x .

Hence no solution is possible.

Coool solution!

Sung Moo Hong - 7 years, 2 months ago
Sung Moo Hong
Mar 20, 2014

X^3 = -p(y^3+z^3 p) So p l x^3 then p l x Same way p l y , p l z X^3 + y^3 p + z^3 p^2 = (Pk)^3 + (pl)^3 p + (pm)^3*p^2

(k)^3 + (l)^3 p + (m)^3 p^2 is same type pf above equation. so, X=p^a y=p^b z=p^c. X^3+ y^3 p +z^3 p^2 = p^{3a}+p^{3b+1}+p^{3c+2} =0 Divide all terms with p^{3b+1}, p^{3(a-b)-1}+ p^{3(c-b)+1} = -1. Above equation is always fault whatever a,b,c is. So there are no solution (x,y,z,p).

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