A full version app for Android, by hosinoatusi.

What are you waiting for? Download Mathical Keyboard B and enter all the mathematical symbols and mathematical expressions you want. You don’t have to switch to another keyboard or convert the characters. You can just tap the desired key on the keyboard to enter. 

If you press a key continuously, it switches to a different symbol. If you want to type lowercased Greek letters “α, β, θ, μ, π, σ, ω” you just have to long-press the buttons, and it switches to uppercase letters “Α, Β, Θ, Μ, Π, Σ, Ω.”

How to enable math symbols: