Assignment:
Use Euler's Criterion to determine whether a is a quadratic residue modulo p in each of the following instances:
(a) a = 2, p = 5; (b) a = 4, p = 7;
(c) a = 3, p = 11; (d) a = 6, p = 13.
Provide complete and step by step solution for the question and show calculations and use formulas.