The millennium prize problems You can also check the book "The Millennium problems: the seven greatest unsolved mathematical puzzles of our time" by K. Devlin. Math stacks QA93 .D485 2002
Truth tables for the main connectives
Handout on how to negate a statement
Slides on number of rows of a truth table
Slides on gcd and Euclidean algorithm
Slides on liear Diophantine equations
Slides on linear congruences part 1