Solve $x^2 - 3x - 5 \equiv 0 \pmod{343}$

Asked Jun 15 '17 at 19:18

Active Jun 15 '17 at 19:18

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I'm trying to solve $$x^2 - 3x - 5 \equiv 0 \pmod{343}$$

I've tried to look up similar problems on the Internet [1] but most of them use Hensel's Lemma which I cannot use as it wasn't introduced by my professor.

The problem is that I cannot transform $x^2 - 3x - 5$ into any familiar form I'd be able to solve. Plus, I've found that $x\in\{1,2\}$ for $x^2 - 3x - 5 \equiv 0 \pmod{7}$. How can I approach this congruence?

asked Jun 15 '17 at 19:18

Mateusz Piotrowski

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Try completing the square! – JavaMan Jun 15 '17 at 19:21
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@JavaMan Good hint! I forgot about it. – Mateusz Piotrowski Jun 15 '17 at 19:22
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So, in the first case $x \equiv 1\pmod{7}$, write $x = 1+7a$, so then $343 \mid (49a^2 + 14a + 1) - (3 + 21a) - 5 = 49a^2 - 7a - 7$. Thus $49 \mid 7a^2 - a - 1$ so $7 \mid -a-1$ which tells you $a \equiv 6\pmod{7}$, and therefore $x \equiv 43 \pmod{49}$. Then, similarly, you can write $x = 43 + 49b$ and substitute to figure out what $x$ is congruent to $\pmod{343}$. – Daniel Schepler Jun 15 '17 at 19:25
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Then repeat for the case $x \equiv 2\pmod{7}$. – Daniel Schepler Jun 15 '17 at 19:26
@DanielSchepler I got it. Thank you. – Mateusz Piotrowski Jun 15 '17 at 19:40
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See my answer here for how to solve quadratic congruences in general. In this case, though, we get $(2x-3)^2\equiv 29\pmod{7^3}$, which doesn't seem to make the problem easier. $29$ is a quadratic residue mod $7^3$. – user236182 Jun 15 '17 at 20:25

Solve $x^2 - 3x - 5 \equiv 0 \pmod{343}$

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