Modular arithmetic

• How to Compute the Modular Inverse in Elliptic-Curve Addition
  For elliptic-curve addition over a prime field, invert the coordinate difference modulo p with the extended Euclidean algorithm—not ordinary division.
• What Is the Divisibility-by-7 Rule, and Why Does It Work?
  Test divisibility by 7 by subtracting twice the last digit from the remaining number; congruence arithmetic explains why the rule works.