Legendre’s Symbol

May 1, 2012

The Legendre Symbol and its cousin the Jacobi Symbol are used in modular arithmetic to determine if a number a is a quadratic residue to the modulus m. A number a is a quadratic residue if there exists a number x such that x2a (mod m) and is defined only when a and m are co-prime. For instance, a2 (mod 7) for a from 0 to 6 is the list 02 (mod 7) = 0, 12 (mod 7) = 1, 22 (mod 7) = 4, 32 (mod 7) = 2, 42 (mod 7) = 2, 52 (mod 7) = 4, and 62 (mod 7) = 1, so the quadratic residues of 7 are 1, 2 and 4 (0 is excluded because it isn’t co-prime to 7). The jacobi symbol considers any odd modulus; the legendre symbol is limited to odd prime moduli. The symbols are usually written in parentheses with a over m, like this: \left( a \atop m \right). Sometimes the symbol is written with a horizontal rule between the a and m, and sometimes it is written on a single line as (a / m).

The legendre/jacobi symbol can be calculated according to the following three termination rules:

1. (0 / m) = 0

2. (1 / m) = 1

3. (2 / m) = −1 if m mod 8 ∈ {3, 5} or 1 if m mod 8 ∈ {1, 7}

and the following three reduction rules:

4. [reducing factors of 2] (2a / m) = (2 / m) × (a / m)

5. [reducing modulo m] (a / m) = (a (mod m) / m) if am or a < 0

6. [reducing odd co-primes a and m] (a /m) = −(m / a) if am ≡ 3 (mod 4), or (m / a) otherwise

Thus, the legendre/jacobi symbol is 1 if a is a quadratic residue, -1 if a is not a quadratic residue, and 0 if a and m are not co-prime.

Our various prime-number programs have used a definition of the legendre/jacobi symbol that doesn’t work; for some values of a and m it returned wrong results, and for other values of a and m it entered an infinite loop. This exercise fixes the problem.

Your task is to write a function that calculates the legendre/jacobi symbol using the rules given above. When you are finished, you are welcome to read or run a suggested solution, or to post your own solution or discuss the exercise in the comments below.

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