Polynomial Diophantine Equation 1

How many integral solutions are there for the following equation?

x 6 + 3 x 4 + 6 x 2 + 5 = y 3 \large{x^6+3x^4+6x^2+5=y^3}


This is a part of the set Polynomial Diophantine Equations


The answer is 0.

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1 solution

Ishan Singh
May 8, 2016

Given : x 6 + 3 x 4 + 6 x 2 + 5 = y 3 x^6+3x^4+6x^2+5=y^3

Consider the L.H.S. \text{L.H.S.}

x 6 + 3 x 4 + 6 x 2 + 5 x^6+3x^4+6x^2+5

= x 6 + 3 x 4 + 3 x 2 + 1 + 3 x 2 + 3 + 1 = x^6+3x^4+3x^2 + 1 + 3x^2 +3 +1

= ( x 2 + 1 ) 3 + 3 ( x 2 + 1 ) + 1 = (x^2+1)^3 +3(x^2+1) + 1

Let t = x 2 + 1 ; t > 0 t=x^2+1 \ ; \ t>0

t 3 + 3 t + 1 = y 3 \implies t^3 + 3t +1 =y^3

Note that t 3 < t 3 + 3 t + 1 < ( t + 1 ) 3 t^3 < t^3+3t+1 < (t+1)^3

But an integral cube cannot exist in between two consecutive integral cubes.

Number of solutions = 0 \implies \text{Number of solutions} = \boxed{0}

Sweet and simple!

Swapnil Das - 5 years, 1 month ago

Nice explanation ..+1

Rishabh Tiwari - 5 years, 1 month ago

Nice question+solution! +1

Nihar Mahajan - 5 years, 1 month ago

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