HappyKey

HappyKey provides you with a simple keyboard with many useful math symbols. It will fulfill all of your math notation needs with numbers, superscripts, pi, math operators and relations, infinity, and beyond.

Install the app first, then to setup the keyboard go to Settings > General > Keyboard > Keyboards > Add New Keyboard > HappyKey

Note: This is not a calculator. This is an additional keyboard that makes it easier to type math notation.

Credit for the keyboard functionality and design goes to the open source Tasty Imitation Keyboard by Alexei Baboulevitch (GitHub user archagon).