Categories Foundations Defined Piece by Piece - Proper Subset Defined Piece by Piece - Subset Mathematical Induction Number Theory Mathematical Induction Geometry The Square Root of 2 Is Irrational Algebra The Square Root of 2 Is Irrational Definitions Defined Piece by Piece - Proper Subset Defined Piece by Piece - Subset Set Theory Defined Piece by Piece - Proper Subset Defined Piece by Piece - Subset