#### D.12.3.21 quadraticSieve

Procedure from library `crypto.lib` (see crypto_lib).

Usage:
  quadraticSieve(n,c,B,k); n to be factorized, [-c,c] the sieve-intervall, B a list of primes,
  k for using the first k elements in B

Return:
  a list of factors of n or the message: no divisor found

Note:
  The idea being used is to find x,y such that x^2=y^2 mod n then gcd(x-y,n) can be a proper divisor of n

Example:

    LIB "crypto.lib";
    list L=primList(5000);
    quadraticSieve(7429,3,L,4);
    ==> 17
    quadraticSieve(1241143,100,L,50);
    ==> 547

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