When DF parameter was high, the CDF returned NaN value. This was resolved by using the GammaLowerRegularized function straight from the CDF instead of division and multipliciation by Gamma function.

src/Numerics/Distributions/ChiSquared.cs

@@ -356,7 +356,7 @@ namespace MathNet.Numerics.Distributions
                 throw new ArgumentException(Resources.InvalidDistributionParameters);
             }
 
-            return SpecialFunctions.GammaLowerIncomplete(freedom/2.0, x/2.0)/SpecialFunctions.Gamma(freedom/2.0);
+            return SpecialFunctions.GammaLowerRegularized(freedom/2.0, x/2.0);
         }
 
         /// <summary>