I wanted too use BigInteger for my Boo script, but for some reason, I always got the error: >BCE0021: Namespace ‘System.Numerics’ not found, maybe you forgot to add an assembly reference?<
I thought it was because of Boo, so I tried the same with C# and I got the same problem: >error CS0234: The type or namespace name Numerics' does not exist in the namespace System’. Are you missing an assembly reference?< So what is the problem here? I didn’t change any libarys 
System.Numerics namespace is not part of the compatibility list and therefore is not supported in unity.