High School Mathematics Extensions/Further Modular Arithmetic/Project/Finding the Square Root

Further Modular Arithmetic
Multiplicative Group and Discrete Log
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*Finding the square root*


Legendre Symbol


.. to be expanded There is actually a simple way to determine whether a is square

Let g be a generator of G where G is the multiplicative group mod p. Since all the squares form a group therefore, if a is a square, then


and if a is not a square then


we shall use these facts in the next section. .. to be expanded

*Finding the square root*


We aim to describe a way to find a square root in mod m. Let's start with the simplest case, where p is prime. In fact, for square root finding, the simplest case also happens to be the hardest.

If p ≡ 3 (mod 4) then it is easy to find a square root. Just note that if a has a square root then


So let us consider primes equivalent to 1 mod 4. Suppose we can find the square root of a mod p, and let


With the above information, we can find the square of a mod p2. We let


we want x2 ≡ a (mod p2), so


for some k as   so  , we see that


so if we need to find   such that  , we simply need to make   the subject


..generalisation ..example

..method for finding a sqr root mod p