One of the incentives to tackle the Project Euler problems has been the opportunity to build a library of functions usable outside of the PE questions themselves. The first few functions tackle basic arithmetic for numbers that neither Excel nor VBA support natively. In Excel the limit is 15 digits and in VBA the Decimal data type supports 29 digits. But, to go beyond that one must roll their own functions. And the Large Number Arithmetic module is my preliminary stab at it.