MathHandbook can help.
It is an online graphic calculator and computer algebra system. It can perform exact, numeric, symbolic and graphic computation, e.g. arbitrary-precision calculation, solve equation, plot data and user-defined functions, linear regression, symbolic differentation and integration, pattern-match. It is a programming language, in which you can define conditional, case, piecewise, recursive, multi-value functions and procedures, derivatives, integrals and rules.
It can run on iPhone and any mobile with Internet