Welcome to the Formula Compiler. The compiler will generate a mathematical formula for any function which you can program. Rather than bore you with the details of the syntax of the programming language, I recommend you look at examples. You can select an example program from the dropdown menu, and click "Convert" to get its mathematical formula. Copying the syntax in the examples, you can program whatever function you desire, and the Formula Compiler will produce a mathematical formula for it, using only basic arithmetic and summation notation.
Read more about the compiler here.