When I first tried to compile the program it complained that it couldn't find the Numerics.dll, so I had to use the Ubuntu Software Center to install "libmono-system-numerics4.0-cil".
After that I had to figure out where it put the file I needed:
locate System.Numerics.dllAnd finally I was able to compile and run the program:
gmcs -r:/usr/lib/mono/4.0/System.Numerics.dll Test.cs
./Test.exeIt did the trick. Now I am ready to try to do larger projects using C# and dotnet on Linux.
Here is a link to an example of a hello world program: http://www.mono-project.com/Mono_Basics