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Bounded Quadratic Congruence:

Instance: Three positive integers $a$, $b$ and $c$.

Question: Is there a positive integer $x<c$ such that $x^{2} \equiv a \, (mod \ b)$?

Bounded Quadratic Congruence is $NP$-$complete$ [1].

Prime Bounded Quadratic Congruence:

Instance: Three positive integers $a$, $b$ and $c$ such that $b$ is a prime number.

Question: Is there a positive integer $x<c$ such that $x^{2} \equiv a \, (mod \ b)$?

Is this problem $NP$-$complete$ as well?

Reference:

[1] Kenneth L. Manders and Leonard M. Adleman, NP-complete decision problems for binary quadratics, Journal of Computer and System Sciences 16 (1978), no. 2, pp. 168–184.

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    $\begingroup$ The inputs $a$, $b$, and $c$ are given in binary right? :) $\endgroup$
    – Michael Wehar
    Jun 15, 2018 at 19:11
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    $\begingroup$ Yes, they are given in binary encoding... $\endgroup$
    – Frank Vega
    Jun 15, 2018 at 19:18
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    $\begingroup$ Michael Wehar, I think this problem could be solved in randomized polynomial time. Look at this:en.wikipedia.org/wiki/Tonelli%E2%80%93Shanks_algorithm $\endgroup$
    – Frank Vega
    Jun 15, 2018 at 19:20
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    $\begingroup$ Therefore, Prime Bounded Quadratic Congruence will be in NP-complete if and only if RP=NP. What do you think? $\endgroup$
    – Frank Vega
    Jun 15, 2018 at 19:22
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    $\begingroup$ Yes, the problem is solvable in randomized polynomial time. So if you know this, what is the point of the question? $\endgroup$
    – Emil Jeřábek
    Jun 15, 2018 at 22:34

1 Answer 1

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This problem can be solved in randomized polynomial time. Look at this:

https://en.wikipedia.org/wiki/Tonelli%E2%80%93Shanks_algorithm

Therefore, if Prime Bounded Quadratic Congruence would be in NP-complete, then RP = NP.

Consequently, the answer of this question is an outstanding problem in complexity theory, so it is not worth to ask it here...

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