Euler’s identity is a mathematical beauty that ties five fundamental mathematical constants in mathematics together. In the equation, we have *e*, the base of natural logarithms, *i,* the imaginary unit, *π, *the ratio of the circumference of a circle to its diameter. This identity shows a profound connection between the most fundamental numbers in mathematics.

## Math Textbook with Cat Pictures

Sheldon Axler is a funny math professor and he put his cats' pictures into his math textbook, Precalculus: A Prelude to Calculus.