This is an example of a library. You may see a list of other libraries used on Rosetta Code at Category:Solutions by Library.
Math.NET is a mathematical opensource toolkit written in C# for the Microsoft .Net platform. Math.NET aims to provide a self-contained clean framework for both numerical scientific and symbolic algebraic computations. The project is covered mostly under the MIT/X11 license with some optional packages under the GPL or LGPL.
It can be downloaded from http://www.mathdotnet.com/