Thursday, 14 July 2016

elementary number theory - How many solutions does a quadratic congruence have?



Given x2182 mod 727, how many solutions mod 727 does it have? Note 727 is prime and 182=2713.



So I know this is soluble computing (182727), but how do I determine the number of solutions? I know how to do this for linear congruences but not sure how to do it for quadratic.


Answer




7273mod so the computation of \pm 182^\frac{727+1}{4}=\pm182^{182} in modulo 727 will give you the desired solutions. Incidentally, the set of solutions are \{363,364\}.



As another user alludes in his comment; since \Bbb Z_{727} is a field; the equation x^2\equiv a\bmod 727 will have at most two solutions.


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