BitContradiction improvements.

PerfectNumbers.Core/Pow2Provider.cs

@@ -70,6 +70,7 @@ namespace PerfectNumbers.Core
 			61 => BigIntegerNumbers.Pow2Exact61,
 			62 => BigIntegerNumbers.Pow2Exact62,
 			63 => BigIntegerNumbers.Pow2Exact63,
+			_ when i >= 0 => System.Numerics.BigInteger.One << i,
 			_ => throw new ArgumentOutOfRangeException(nameof(i), $"Parameter value {i} is incorrect"),
 		};
 
@@ -140,7 +141,8 @@ namespace PerfectNumbers.Core
 			62 => BigIntegerNumbers.Pow2MinusOne62,
 			63 => BigIntegerNumbers.Pow2MinusOne63,
 			1024 => BigIntegerNumbers.Pow2MinusOne1024,
+			_ when i >= 0 => (System.Numerics.BigInteger.One << i) - System.Numerics.BigInteger.One,
 			_ => throw new ArgumentOutOfRangeException(nameof(i), $"Parameter value {i} is incorrect"),
 		};
     }
-}
+}