PocketCAS is a free interface to the open-source Computer Algebra System giac/xcas.

xcas enables you to quickly solve complex algebraic problems, e.g. symbolic and numeric integration and derivation of non-trivial functions, solving differential equations etc. It has also a huge support for problems of linear algebra, like solving linear equation systems, multiplying and inverting matrices, finding eigenvalues (and eigenvectors, of course) and much more. If you do statistics or number theory, you will like xcas’ great functionality in those areas, too!

Changes:

Added images for the following functions: x^(-1), x^y, e^x, sum, product
Improved images for the following functions: sqrt, integral, pi
Some internal optimizations regarding the XML keyboard. Please tell me whether you notice changes in performance and memory consumption (good or bad)
Greek keys and keys for single-letter variables for easier variable input (located in the “helper” section)
Added more mathematical symbols to the keyboard rather than mere text. This benefits WM 2003 users especially, as they now get some icons rather than mere text buttons.
Improved options for keyboard font sizes (different font sizes for individual sections and buttons)
text size of many keyboard buttons has been improved for even better usability

http://forum.xda-developers.com/showthread.php?t=415169