Wow! This seems amazingly complete for an iOS/OS X-only app that I've never heard of before today. Is it based on some preexisting algebra system, or totally from scratch? Regardless, it's very impressive.
It's from scratch C# code using Xamarin to build the UI layers for iOS/OS X. Well, I also use a parser generator jayc which is based on jay which is a yacc cc. Everything else is mine. It's a pretty standard Expression tree implementation with a symbolic interpreter.
Awesome product, I just bought it for my iPhone. Given that it's C# already, any plans for a windows version? I often do these types of calculations in excel at work, but this would be so handy.