# Diophantine equation with quartic polynomial

What are all integral solutions to $$y^2=x^4+x^3+x^2+x+1$$

The RHS could become $$(x^2-x+1)(x^2+x+1)+x(x^2+1)$$ or $$\frac{x^5-1}{x-1}$$ I have no idea how to manipulate the equation into something useful or what the first step should be. Also, A quartic diophantine equation looks useful, but none of the answers completely solve the question? Thanks!

If $$x \gt 0$$, the only solution is $$x=3, y=11$$.

Note: Wolfy was used extensively getting this answer.

$$y^2 =x^4+x^3+x^2+x+1$$

$$y^2 =x^4+x^3+x^2+x+1 \gt x^4 \implies y > x^2$$.

$$(x+1)^4 =x^4+4x^4+,,,$$ so $$y^2 \lt (x+1)^4$$ or $$y < (x+1)^2$$.

$$(x^2+x/2+3/8)^2 =x^4 + x^3 + x^2 + (3 x)/8 + 9/64 \lt y^2$$ so $$y > x^2+x/2+3/8$$.

$$(x^2+x/2+1)^2 =x^4 + x^3 + (9 x^2)/4 + x + 1 \gt y^2$$ so $$y < x^2+x/2+1$$.

If $$x = 2n$$ then $$4n^2+n+3/8 \lt y \lt 4n^2+n+1$$, so there can be no such integer $$y$$.

If $$x = 2n+1$$ then $$x^2+x/2+3/8 =4n^2+4n+1+n+1/2+3/8 =4n^2+5n+15/8$$ and $$x^2+x/2+1 =4n^2+4n+1+n+1/2+1 =4n^2+5n+5/2$$ so $$y = 4n^2+5n+2$$.

But $$y^2 =16 n^4 + 40 n^3 + 41 n^2 + 20 n + 4$$ and $$x^4+x^3+x^2+x+1 =\dfrac{x^5-1}{x-1} =\dfrac{(2n+1)^5-1}{2n} =16 n^4 + 40 n^3 + 40 n^2 + 20 n + 5$$ and the difference is $$n^2-1$$ so they are never equal unless $$n = 1$$ so $$x = 3$$.

• A minor point, but I think your start of "If $x \gt 0$, ..." should be something like "If $x,y \gt 0$, ..." instead since otherwise, for example, $x = 3, y = -11$ is also a solution. Oct 22 '19 at 1:56
• Very good solution! I believe $n=-1$ yields another case, right? Oct 22 '19 at 16:52
• @Baker013273213 Change $x$ to $-z$ and assume $z>0$. $z$ cannot be even, otherwise $$(z^2-z/2)^2 < z^4-z^3+z^2-z+1=y^2 < (z^2-z/2+1)^2$$ With $z$ odd, we have $$(z^2 - (z+1)/2)^2 < z^4-z^3+z^2-z+1=y^2 < (z^2 - (z+1)/2+1)^2$$ if $z > 1$. So only possibility is $z=1$ giving $x=-1$. Oct 23 '19 at 2:59
• You should make this an answer. Oct 23 '19 at 14:35