added sanity checks

1-tests/Directory.Build.props

@@ -18,4 +18,9 @@
     </PackageReference>
   </ItemGroup>
 
+  <ItemGroup>
+      <Using Include="System.Numerics"/>
+      <Using Include="Shouldly"/>
+      <Using Include="Xunit"/>
+  </ItemGroup>
 </Project>

1-tests/Just.PreciseMath.Tests/ArithmeticRangeTests.cs

@@ -1,6 +1,3 @@
-using Shouldly;
-using Xunit;
-
 namespace Just.PreciseMath.Tests;
 
 public class ArithmeticRangeTests

1-tests/Just.PreciseMath.Tests/DoubleDoubleArithmeticTests.cs

@@ -1,7 +1,3 @@
-using System.Numerics;
-using Shouldly;
-using Xunit;
-
 namespace Just.PreciseMath.Tests;
 
 public class DoubleDoubleArithmeticTests
@@ -332,6 +328,53 @@ public class DoubleDoubleArithmeticTests
         Check(6.0 / new DoubleDouble(2.0), 3.0, 0.0);
     }
 
+    [Theory]
+    [InlineData(2.0)]
+    [InlineData(3.0)]
+    public void ScalarAdditionAndSubtractionRetainTheMiddleSumResidual(double magnitude)
+    {
+        // Exact Fraction arithmetic on the stored E components confirms that
+        // all signed E +/- 2 and E +/- 3 results fit exactly in two components.
+        // Compare exact dyadic sums, not another overload or a rounded tolerance.
+        foreach (double valueSign in new[] { -1.0, 1.0 })
+        {
+            foreach (double scalarSign in new[] { -1.0, 1.0 })
+            {
+                DoubleDouble value = valueSign * DoubleDouble.E;
+                double scalar = scalarSign * magnitude;
+                BigInteger expectedSum = Units(value) + Units(scalar);
+                BigInteger expectedDifference = Units(value) - Units(scalar);
+                Units(value + scalar).ShouldBe(expectedSum);
+                Units(scalar + value).ShouldBe(expectedSum);
+                Units(value - scalar).ShouldBe(expectedDifference);
+                Units(scalar - value).ShouldBe(-expectedDifference);
+            }
+        }
+    }
+
+    [Theory]
+    [InlineData("+")]
+    [InlineData("-")]
+    public void ScalarSumsAndDifferencesMatchExpansionBitsAcrossRanges(string operation)
+    {
+        // Bitwise compatibility; independent exact-rational accuracy checks are separate.
+        (double[] scalars, DoubleDouble[] values) = ScalarArithmeticCompatibilityCases();
+        foreach (DoubleDouble value in values)
+        {
+            foreach (double scalar in scalars)
+            {
+                DoubleDouble expanded = new(scalar);
+                DoubleDouble expected = operation == "+" ? value + expanded : value - expanded;
+                DoubleDouble actual = operation == "+" ? value + scalar : value - scalar;
+                CheckBoundary(actual, expected.High, expected.Low);
+
+                DoubleDouble expectedReverse = operation == "+" ? expanded + value : expanded - value;
+                DoubleDouble actualReverse = operation == "+" ? scalar + value : scalar - value;
+                CheckBoundary(actualReverse, expectedReverse.High, expectedReverse.Low);
+            }
+        }
+    }
+
     [Fact]
     public void ExpansionCancellationRetainsExactComponentsAcrossTheAdditionGuard()
     {
@@ -579,6 +622,25 @@ public class DoubleDoubleArithmeticTests
         }
     }
 
+    [Fact]
+    public void DivisionOverloadsPreserveComponentBitsAcrossRangesAndSpecialValues()
+    {
+        // Compatibility, not an accuracy oracle. GeneralArithmeticMeetsExactRationalBound
+        // separately checks complete results against exact rational bounds.
+        (double[] scalars, DoubleDouble[] values) = ScalarArithmeticCompatibilityCases();
+        foreach (DoubleDouble value in values)
+        {
+            foreach (double scalar in scalars)
+            {
+                DoubleDouble expectedQuotient = value / new DoubleDouble(scalar);
+                CheckBoundary(value / scalar, expectedQuotient.High, expectedQuotient.Low);
+
+                DoubleDouble expectedReverseQuotient = new DoubleDouble(scalar) / value;
+                CheckBoundary(scalar / value, expectedReverseQuotient.High, expectedReverseQuotient.Low);
+            }
+        }
+    }
+
     [Fact]
     public void DivisionRetainsSubnormalCorrectionsWithOrdinaryHighComponents()
     {
@@ -597,6 +659,38 @@ public class DoubleDoubleArithmeticTests
         }
     }
 
+    [Theory]
+    [InlineData(-1000)]
+    [InlineData(-1021)]
+    [InlineData(-1022)]
+    [InlineData(-1023)]
+    public void ScalarDivisionPreservesSparseCorrectionProductsNearUnderflow(int lowExponent)
+    {
+        // At -1000, the second product is normal but its exact residual is
+        // (962132647665515 / 1073741824) * epsilon, which rounds to 896056 * epsilon.
+        // Derived from exact binary64 rational products; the other rows bracket
+        // the correction product's normal/subnormal transition.
+        foreach (double numeratorSign in new[] { -1.0, 1.0 })
+        {
+            foreach (double denominatorSign in new[] { -1.0, 1.0 })
+            {
+                foreach (double lowSign in new[] { -1.0, 1.0 })
+                {
+                    double low = Math.ScaleB(lowSign, lowExponent);
+                    double denominator = denominatorSign * 1.1;
+                    DoubleDouble numerator = DoubleDouble.FromComponents(numeratorSign * 1.1, low);
+                    DoubleDouble actual = numerator / denominator;
+                    DoubleDouble expanded = numerator / new DoubleDouble(denominator);
+                    CheckBoundary(actual, expanded.High, expanded.Low);
+                    // Exact quotient = +/-1 + low/denominator. Its high cannot
+                    // change here; binary64 division independently rounds the low.
+                    CheckBoundary(actual, numeratorSign / denominatorSign, low / denominator);
+                    AssertRelative(actual, Units(numerator) << 1074, Units(denominator));
+                }
+            }
+        }
+    }
+
     [Fact]
     public void SubnormalProductsWithLargeNormalsRetainExactResultsInBothOrders()
     {
@@ -884,6 +978,39 @@ public class DoubleDoubleArithmeticTests
         }
     }
 
+    private static (double[] Scalars, DoubleDouble[] Values) ScalarArithmeticCompatibilityCases()
+    {
+        List<double> scalars = [0.0, -0.0, 1.0, -1.0, 2.0, -2.0, 3.0, -3.0, 100.0, -100.0,
+            double.MaxValue, double.MinValue, double.PositiveInfinity,
+            double.NegativeInfinity, double.NaN];
+        List<DoubleDouble> values = [DoubleDouble.E, -DoubleDouble.E];
+        // Include addition/division dispatch edges, exponent extremes, dense/sparse
+        // lows of either sign, and significands on either side of a binade.
+        int[] exponents = [-1074, -1022, -451, -450, -1, 0, 1, 450, 451, 1020, 1021, 1023];
+        double[] significands = [1.0, Math.BitIncrement(1.0), 1.5, Math.BitDecrement(2.0)];
+        foreach (int exponent in exponents)
+        {
+            foreach (double significand in significands)
+            {
+                foreach (double sign in new[] { -1.0, 1.0 })
+                {
+                    double high = Math.ScaleB(sign * significand, exponent);
+                    scalars.Add(high);
+                    double denseLow = Math.ScaleB(1.0, exponent - 54);
+                    foreach (double low in new[] { 0.0, denseLow, -denseLow, double.Epsilon, -double.Epsilon })
+                    {
+                        values.Add(DoubleDouble.FromComponents(high, low));
+                    }
+                }
+            }
+        }
+        foreach (double scalar in scalars)
+        {
+            values.Add(new DoubleDouble(scalar));
+        }
+        return (scalars.ToArray(), values.ToArray());
+    }
+
     private static void AssertDivisionMatchesPrevious(DoubleDouble left, DoubleDouble right)
     {
         // Freeze the pre-specialization expression. The unchanged public operators

1-tests/Just.PreciseMath.Tests/DoubleDoubleBoundaryTests.cs

@@ -1,7 +1,3 @@
-using System.Numerics;
-using Shouldly;
-using Xunit;
-
 namespace Just.PreciseMath.Tests;
 
 public class DoubleDoubleBoundaryTests

1-tests/Just.PreciseMath.Tests/DoubleDoubleCbrtTests.cs

@@ -1,7 +1,3 @@
-using System.Numerics;
-using Shouldly;
-using Xunit;
-
 namespace Just.PreciseMath.Tests;
 
 public class DoubleDoubleCbrtTests

1-tests/Just.PreciseMath.Tests/DoubleDoubleComparisonTests.cs

@@ -1,6 +1,3 @@
-using Shouldly;
-using Xunit;
-
 namespace Just.PreciseMath.Tests;
 
 public class DoubleDoubleComparisonTests

1-tests/Just.PreciseMath.Tests/DoubleDoubleConversionTests.cs

@@ -1,7 +1,3 @@
-using System.Numerics;
-using Shouldly;
-using Xunit;
-
 namespace Just.PreciseMath.Tests;
 
 public class DoubleDoubleConversionTests

1-tests/Just.PreciseMath.Tests/DoubleDoubleExponentialFunctionsTests.cs

@@ -1,8 +1,5 @@
 using System.Globalization;
-using System.Numerics;
 using Just.PreciseMath.Tests.ReferenceData;
-using Shouldly;
-using Xunit;
 
 namespace Just.PreciseMath.Tests;
 

1-tests/Just.PreciseMath.Tests/DoubleDoubleFormattingTests.cs

@@ -1,6 +1,4 @@
 using System.Globalization;
-using Shouldly;
-using Xunit;
 
 namespace Just.PreciseMath.Tests;
 

1-tests/Just.PreciseMath.Tests/DoubleDoubleHypotTests.cs

@@ -1,7 +1,3 @@
-using System.Numerics;
-using Shouldly;
-using Xunit;
-
 namespace Just.PreciseMath.Tests;
 
 public class DoubleDoubleHypotTests

1-tests/Just.PreciseMath.Tests/DoubleDoubleNumberStylesTests.cs

@@ -1,7 +1,4 @@
 using System.Globalization;
-using System.Numerics;
-using Shouldly;
-using Xunit;
 
 namespace Just.PreciseMath.Tests;
 

1-tests/Just.PreciseMath.Tests/DoubleDoubleParsingTests.cs

@@ -1,7 +1,4 @@
 using System.Globalization;
-using System.Numerics;
-using Shouldly;
-using Xunit;
 
 namespace Just.PreciseMath.Tests;
 

1-tests/Just.PreciseMath.Tests/DoubleDoubleRepresentationTests.cs

@@ -1,6 +1,3 @@
-using Shouldly;
-using Xunit;
-
 namespace Just.PreciseMath.Tests;
 
 public class DoubleDoubleRepresentationTests

1-tests/Just.PreciseMath.Tests/DoubleDoubleRootFunctionsTests.cs

@@ -1,6 +1,3 @@
-using Shouldly;
-using Xunit;
-
 namespace Just.PreciseMath.Tests;
 
 public class DoubleDoubleRootFunctionsTests

1-tests/Just.PreciseMath.Tests/DoubleDoubleRootNTests.cs

@@ -1,8 +1,5 @@
 using System.Globalization;
-using System.Numerics;
 using Just.PreciseMath.Tests.ReferenceData;
-using Shouldly;
-using Xunit;
 
 namespace Just.PreciseMath.Tests;
 

1-tests/Just.PreciseMath.Tests/DoubleDoubleSignedNumberTests.cs

@@ -1,7 +1,3 @@
-using System.Numerics;
-using Shouldly;
-using Xunit;
-
 namespace Just.PreciseMath.Tests;
 
 public class DoubleDoubleSignedNumberTests

1-tests/Just.PreciseMath.Tests/DoubleDoubleSpanFormattingTests.cs

@@ -1,6 +1,4 @@
 using System.Globalization;
-using Shouldly;
-using Xunit;
 
 namespace Just.PreciseMath.Tests;
 

1-tests/Just.PreciseMath.Tests/DoubleDoubleSpecialValueTests.cs

@@ -1,7 +1,4 @@
 using System.Globalization;
-using System.Numerics;
-using Shouldly;
-using Xunit;
 
 namespace Just.PreciseMath.Tests;
 

1-tests/Just.PreciseMath.Tests/DoubleDoubleTests.cs

@@ -1,7 +1,3 @@
-using System.Numerics;
-using Shouldly;
-using Xunit;
-
 namespace Just.PreciseMath.Tests;
 
 public class DoubleDoubleTests

1-tests/Just.PreciseMath.Tests/GenericConversionTests.cs

@@ -1,7 +1,4 @@
 using System.Diagnostics.CodeAnalysis;
-using System.Numerics;
-using Shouldly;
-using Xunit;
 
 namespace Just.PreciseMath.Tests;
 

1-tests/Just.PreciseMath.Tests/PreciseMathExpTests.cs

@@ -1,8 +1,5 @@
 using System.Globalization;
-using System.Numerics;
 using Just.PreciseMath.Tests.ReferenceData;
-using Shouldly;
-using Xunit;
 
 namespace Just.PreciseMath.Tests;
 

1-tests/Just.PreciseMath.Tests/PreciseMathInvSqrtTests.cs

@@ -1,7 +1,3 @@
-using System.Numerics;
-using Shouldly;
-using Xunit;
-
 namespace Just.PreciseMath.Tests;
 
 public class PreciseMathInvSqrtTests

1-tests/Just.PreciseMath.Tests/PreciseMathLogTests.cs

@@ -1,8 +1,5 @@
 using System.Globalization;
-using System.Numerics;
 using Just.PreciseMath.Tests.ReferenceData;
-using Shouldly;
-using Xunit;
 
 namespace Just.PreciseMath.Tests;
 

1-tests/Just.PreciseMath.Tests/PreciseMathPowTests.cs

@@ -1,7 +1,3 @@
-using System.Numerics;
-using Shouldly;
-using Xunit;
-
 namespace Just.PreciseMath.Tests;
 
 public class PreciseMathPowTests

1-tests/Just.PreciseMath.Tests/PreciseMathRealPowSpecialTests.cs

@@ -1,6 +1,3 @@
-using Shouldly;
-using Xunit;
-
 namespace Just.PreciseMath.Tests;
 
 public class PreciseMathRealPowSpecialTests

1-tests/Just.PreciseMath.Tests/PreciseMathRealPowTests.cs

@@ -1,8 +1,5 @@
 using System.Globalization;
-using System.Numerics;
 using Just.PreciseMath.Tests.ReferenceData;
-using Shouldly;
-using Xunit;
 
 namespace Just.PreciseMath.Tests;
 

1-tests/Just.PreciseMath.Tests/PreciseMathReciprocalTests.cs

@@ -1,7 +1,3 @@
-using System.Numerics;
-using Shouldly;
-using Xunit;
-
 namespace Just.PreciseMath.Tests;
 
 public class PreciseMathReciprocalTests

1-tests/Just.PreciseMath.Tests/PreciseMathSqrtTests.cs

@@ -1,7 +1,3 @@
-using System.Numerics;
-using Shouldly;
-using Xunit;
-
 namespace Just.PreciseMath.Tests;
 
 public class PreciseMathSqrtTests

1-tests/Just.PreciseMath.Tests/PreciseMathTests.cs

@@ -1,6 +1,3 @@
-using Shouldly;
-using Xunit;
-
 namespace Just.PreciseMath.Tests;
 
 public class PreciseMathTests

1-tests/Just.PreciseMath.Tests/ReferenceData/ExpReferenceData.cs

@@ -1,5 +1,3 @@
-using Xunit;
-
 namespace Just.PreciseMath.Tests.ReferenceData;
 
 // Generated by generate_exp.py; do not derive expected values from DD arithmetic.

1-tests/Just.PreciseMath.Tests/ReferenceData/ExponentialFunctionsReferenceData.cs

@@ -9,8 +9,6 @@
 // absolute; reference rounding uncertainty is bounded by 2^-350 relative.
 // Tiny corrections to 1/-1 below 120 digits are not component-retention oracles.
 // Signed zeros, infinities, NaNs, and enormous arguments are tested separately.
-using Xunit;
-
 namespace Just.PreciseMath.Tests.ReferenceData;
 
 internal static class ExponentialFunctionsReferenceData

1-tests/Just.PreciseMath.Tests/ReferenceData/LogReferenceData.cs

@@ -1,7 +1,5 @@
 // Generated by generate_log.py; do not hand-edit reference literals.
 // Exact binary64 sums; Decimal.ln at 450/650 digits, rounded to 120 digits.
-using Xunit;
-
 namespace Just.PreciseMath.Tests.ReferenceData;
 
 internal static class LogReferenceData

1-tests/Just.PreciseMath.Tests/ReferenceData/RealPowReferenceData.cs

@@ -1,8 +1,6 @@
 // Generated by generate_real_pow.py; do not hand-edit reference literals.
 // Exact binary64 sums; Decimal ln/exp at 450/650 digits, rounded to 120 digits.
 // Reference uncertainty is explicitly allowed as 2^-350 relative in the tests.
-using Xunit;
-
 namespace Just.PreciseMath.Tests.ReferenceData;
 
 internal static class RealPowReferenceData

1-tests/Just.PreciseMath.Tests/ReferenceData/RootNReferenceData.cs

@@ -1,7 +1,5 @@
 // Generated by generate_rootn.py; do not hand-edit reference literals.
 // Exact component sums; Decimal ln/exp at 450/650 digits, rounded to 120 digits.
-using Xunit;
-
 namespace Just.PreciseMath.Tests.ReferenceData;
 
 internal static class RootNReferenceData

1-tests/Just.PreciseMath.Tests/SanityChecks/DoubleDoubleAdditionChecks.cs

@@ -0,0 +1,27 @@
+namespace Just.PreciseMath.Tests.SanityChecks;
+
+public class DoubleDoubleAdditionChecks
+{
+    public static IEnumerable<TheoryDataRow<DoubleDouble, DoubleDouble, DoubleDouble>> ReferenceCases =>
+    [
+        (new DoubleDouble(1.0), new DoubleDouble(1.0), new DoubleDouble(2.0)),
+        (new DoubleDouble(1.0), new DoubleDouble(2.0), new DoubleDouble(3.0)),
+        (new DoubleDouble(1.25), new DoubleDouble(2.5), new DoubleDouble(3.75)),
+        (new DoubleDouble(65535.0), new DoubleDouble(1.0), new DoubleDouble(65536.0)),
+        (DoubleDouble.Parse("812563124576179134546", System.Globalization.CultureInfo.InvariantCulture), DoubleDouble.Parse("-812563124576179134504", System.Globalization.CultureInfo.InvariantCulture), new DoubleDouble(42.0)),
+        (DoubleDouble.Parse("812563124576179134546", System.Globalization.CultureInfo.InvariantCulture), new DoubleDouble(127.0), DoubleDouble.Parse("812563124576179134673", System.Globalization.CultureInfo.InvariantCulture)),
+        (DoubleDouble.Parse("812563124576179134546", System.Globalization.CultureInfo.InvariantCulture), new DoubleDouble(27.0), DoubleDouble.Parse("812563124576179134573", System.Globalization.CultureInfo.InvariantCulture)),
+        (DoubleDouble.Parse("812563124576179134546", System.Globalization.CultureInfo.InvariantCulture), new DoubleDouble(2.70), DoubleDouble.Parse("812563124576179134548.7", System.Globalization.CultureInfo.InvariantCulture)),
+    ];
+
+    [Theory]
+    [MemberData(nameof(ReferenceCases))]
+    public void AdditionShouldProduceExpectedResults(DoubleDouble left, DoubleDouble right, DoubleDouble expected)
+    {
+        DoubleDouble result = left + right;
+
+        result.High.ShouldBe(expected.High);
+        result.Low.ShouldBe(expected.Low);
+        result.ShouldBe(expected);
+    }
+}

1-tests/Just.PreciseMath.Tests/SanityChecks/DoubleDoubleArithmeticEqualityChecks.cs

@@ -0,0 +1,108 @@
+namespace Just.PreciseMath.Tests.SanityChecks;
+
+public class DoubleDoubleArithmeticEqualityChecks
+{
+    public static IEnumerable<TheoryDataRow<double>> ReferenceCases => [1.0, 2.0, 3.0, 100.0, 0.1, 0.5, 0.333333333333333333, -1.0, 1553.0, 1.0e+10];
+
+    [Theory]
+    [MemberData(nameof(ReferenceCases))]
+    public void AdditionShouldProduceIdenticalResults(double reference)
+    {
+        DoubleDouble testValue = DoubleDouble.E; // any value with full precision range used
+
+        DoubleDouble ddxdd_Result = testValue + (new DoubleDouble(reference));
+        DoubleDouble ddxd_Result = testValue + reference;
+        DoubleDouble dxdd_Result = reference + testValue;
+
+        ddxdd_Result.High.ShouldBe(ddxd_Result.High);
+        ddxdd_Result.Low.ShouldBe(ddxd_Result.Low);
+        ddxdd_Result.ShouldBe(ddxd_Result);
+
+        ddxdd_Result.High.ShouldBe(dxdd_Result.High);
+        ddxdd_Result.Low.ShouldBe(dxdd_Result.Low);
+        ddxdd_Result.ShouldBe(dxdd_Result);
+
+        ddxd_Result.High.ShouldBe(dxdd_Result.High);
+        ddxd_Result.Low.ShouldBe(dxdd_Result.Low);
+        ddxd_Result.ShouldBe(dxdd_Result);
+    }
+
+    [Theory]
+    [MemberData(nameof(ReferenceCases))]
+    public void SubtractionFromShouldProduceIdenticalResults(double reference)
+    {
+        DoubleDouble testValue = DoubleDouble.E; // any value with full precision range used
+
+        DoubleDouble ddxdd_Result = testValue - (new DoubleDouble(reference));
+        DoubleDouble ddxd_Result = testValue - reference;
+
+        ddxdd_Result.High.ShouldBe(ddxd_Result.High);
+        ddxdd_Result.Low.ShouldBe(ddxd_Result.Low);
+        ddxdd_Result.ShouldBe(ddxd_Result);
+    }
+
+    [Theory]
+    [MemberData(nameof(ReferenceCases))]
+    public void SubtractionShouldProduceIdenticalResults(double reference)
+    {
+        DoubleDouble testValue = DoubleDouble.E; // any value with full precision range used
+
+        DoubleDouble ddxdd_Result = (new DoubleDouble(reference)) - testValue;
+        DoubleDouble dxdd_Result = reference - testValue;
+
+        ddxdd_Result.High.ShouldBe(dxdd_Result.High);
+        ddxdd_Result.Low.ShouldBe(dxdd_Result.Low);
+        ddxdd_Result.ShouldBe(dxdd_Result);
+    }
+
+    [Theory]
+    [MemberData(nameof(ReferenceCases))]
+    public void MultiplicationShouldProduceIdenticalResults(double reference)
+    {
+        DoubleDouble testValue = DoubleDouble.E; // any value with full precision range used
+
+        DoubleDouble ddxdd_Result = testValue * (new DoubleDouble(reference));
+        DoubleDouble ddxd_Result = testValue * reference;
+        DoubleDouble dxdd_Result = reference * testValue;
+
+        ddxdd_Result.High.ShouldBe(ddxd_Result.High);
+        ddxdd_Result.Low.ShouldBe(ddxd_Result.Low);
+        ddxdd_Result.ShouldBe(ddxd_Result);
+
+        ddxdd_Result.High.ShouldBe(dxdd_Result.High);
+        ddxdd_Result.Low.ShouldBe(dxdd_Result.Low);
+        ddxdd_Result.ShouldBe(dxdd_Result);
+
+        ddxd_Result.High.ShouldBe(dxdd_Result.High);
+        ddxd_Result.Low.ShouldBe(dxdd_Result.Low);
+        ddxd_Result.ShouldBe(dxdd_Result);
+    }
+
+    [Theory]
+    [MemberData(nameof(ReferenceCases))]
+    public void DivisionShouldProduceIdenticalResults(double reference)
+    {
+        DoubleDouble testValue = DoubleDouble.E; // any value with full precision range used
+
+        DoubleDouble ddxdd_Result = testValue / (new DoubleDouble(reference));
+        DoubleDouble ddxd_Result = testValue / reference;
+
+        ddxdd_Result.High.ShouldBe(ddxd_Result.High);
+        ddxdd_Result.Low.ShouldBe(ddxd_Result.Low);
+        ddxdd_Result.ShouldBe(ddxd_Result);
+    }
+
+    [Theory]
+    [MemberData(nameof(ReferenceCases))]
+    public void DivisionByShouldProduceIdenticalResults(double reference)
+    {
+        DoubleDouble testValue = DoubleDouble.E; // any value with full precision range used
+
+        DoubleDouble ddxdd_Result = (new DoubleDouble(reference)) / testValue;
+        DoubleDouble dxdd_Result = reference / testValue;
+
+        ddxdd_Result.High.ShouldBe(dxdd_Result.High);
+        ddxdd_Result.Low.ShouldBe(dxdd_Result.Low);
+        ddxdd_Result.ShouldBe(dxdd_Result);
+    }
+}

1-tests/Just.PreciseMath.Tests/SanityChecks/DoubleDoubleDivisionChecks.cs

@@ -0,0 +1,29 @@
+namespace Just.PreciseMath.Tests.SanityChecks;
+
+public class DoubleDoubleDivisionChecks
+{
+    public static IEnumerable<TheoryDataRow<DoubleDouble, DoubleDouble, DoubleDouble>> ReferenceCases =>
+    [
+        (new DoubleDouble(6.0), new DoubleDouble(2.0), new DoubleDouble(3.0)),
+        (new DoubleDouble(6.0), new DoubleDouble(3.0), new DoubleDouble(2.0)),
+        (new DoubleDouble(5.0), new DoubleDouble(2.0), new DoubleDouble(2.5)),
+        (new DoubleDouble(1.0), new DoubleDouble(4.0), new DoubleDouble(0.25)),
+        (new DoubleDouble(3.75), new DoubleDouble(1.25), new DoubleDouble(3.0)),
+        (new DoubleDouble(65536.0), new DoubleDouble(1024.0), new DoubleDouble(64.0)),
+        (DoubleDouble.Parse("812563124576179134546", System.Globalization.CultureInfo.InvariantCulture), new DoubleDouble(2.0), DoubleDouble.Parse("406281562288089567273", System.Globalization.CultureInfo.InvariantCulture)),
+        (DoubleDouble.Parse("812563124576179134546", System.Globalization.CultureInfo.InvariantCulture), new DoubleDouble(4.0), DoubleDouble.Parse("203140781144044783636.5", System.Globalization.CultureInfo.InvariantCulture)),
+        (DoubleDouble.Parse("812563124576179134546", System.Globalization.CultureInfo.InvariantCulture), new DoubleDouble(8.0), DoubleDouble.Parse("101570390572022391818.25", System.Globalization.CultureInfo.InvariantCulture)),
+        (DoubleDouble.Parse("812563124576179134546", System.Globalization.CultureInfo.InvariantCulture), DoubleDouble.Parse("812563124576179134546", System.Globalization.CultureInfo.InvariantCulture), new DoubleDouble(1.0)),
+    ];
+
+    [Theory]
+    [MemberData(nameof(ReferenceCases))]
+    public void DivisionShouldProduceExpectedResults(DoubleDouble left, DoubleDouble right, DoubleDouble expected)
+    {
+        DoubleDouble result = left / right;
+
+        result.High.ShouldBe(expected.High);
+        result.Low.ShouldBe(expected.Low);
+        result.ShouldBe(expected);
+    }
+}

1-tests/Just.PreciseMath.Tests/SanityChecks/DoubleDoubleMultiplicationChecks.cs

@@ -0,0 +1,29 @@
+namespace Just.PreciseMath.Tests.SanityChecks;
+
+public class DoubleDoubleMultiplicationChecks
+{
+    public static IEnumerable<TheoryDataRow<DoubleDouble, DoubleDouble, DoubleDouble>> ReferenceCases =>
+    [
+        (new DoubleDouble(1.0), new DoubleDouble(1.0), new DoubleDouble(1.0)),
+        (new DoubleDouble(2.0), new DoubleDouble(3.0), new DoubleDouble(6.0)),
+        (new DoubleDouble(1.25), new DoubleDouble(2.5), new DoubleDouble(3.125)),
+        (new DoubleDouble(-1.25), new DoubleDouble(2.5), new DoubleDouble(-3.125)),
+        (new DoubleDouble(65535.0), new DoubleDouble(65536.0), new DoubleDouble(4294901760.0)),
+        (new DoubleDouble(65536.0), new DoubleDouble(65536.0), new DoubleDouble(4294967296.0)),
+        (DoubleDouble.Parse("812563124576179134546", System.Globalization.CultureInfo.InvariantCulture), new DoubleDouble(2.0), DoubleDouble.Parse("1625126249152358269092", System.Globalization.CultureInfo.InvariantCulture)),
+        (DoubleDouble.Parse("812563124576179134546", System.Globalization.CultureInfo.InvariantCulture), new DoubleDouble(0.5), DoubleDouble.Parse("406281562288089567273", System.Globalization.CultureInfo.InvariantCulture)),
+        (DoubleDouble.Parse("812563124576179134546", System.Globalization.CultureInfo.InvariantCulture), new DoubleDouble(27.0), DoubleDouble.Parse("21939204363556836632742", System.Globalization.CultureInfo.InvariantCulture)),
+        (DoubleDouble.Parse("812563124576179134546", System.Globalization.CultureInfo.InvariantCulture), new DoubleDouble(127.0), DoubleDouble.Parse("103195516821174750087342", System.Globalization.CultureInfo.InvariantCulture)),
+    ];
+
+    [Theory]
+    [MemberData(nameof(ReferenceCases))]
+    public void MultiplicationShouldProduceExpectedResults(DoubleDouble left, DoubleDouble right, DoubleDouble expected)
+    {
+        DoubleDouble result = left * right;
+
+        result.High.ShouldBe(expected.High);
+        result.Low.ShouldBe(expected.Low);
+        result.ShouldBe(expected);
+    }
+}

1-tests/Just.PreciseMath.Tests/SanityChecks/DoubleDoubleSubtractionChecks.cs

@@ -0,0 +1,30 @@
+namespace Just.PreciseMath.Tests.SanityChecks;
+
+public class DoubleDoubleSubtractionChecks
+{
+    public static IEnumerable<TheoryDataRow<DoubleDouble, DoubleDouble, DoubleDouble>> ReferenceCases =>
+    [
+        (new DoubleDouble(2.0), new DoubleDouble(1.0), new DoubleDouble(1.0)),
+        (new DoubleDouble(1.0), new DoubleDouble(2.0), new DoubleDouble(-1.0)),
+        (new DoubleDouble(3.75), new DoubleDouble(2.5), new DoubleDouble(1.25)),
+        (new DoubleDouble(2.5), new DoubleDouble(3.75), new DoubleDouble(-1.25)),
+        (new DoubleDouble(65536.0), new DoubleDouble(1.0), new DoubleDouble(65535.0)),
+        (new DoubleDouble(65536.0), new DoubleDouble(65535.0), new DoubleDouble(1.0)),
+        (DoubleDouble.Parse("812563124576179134673", System.Globalization.CultureInfo.InvariantCulture), new DoubleDouble(127.0), DoubleDouble.Parse("812563124576179134546", System.Globalization.CultureInfo.InvariantCulture)),
+        (DoubleDouble.Parse("812563124576179134546", System.Globalization.CultureInfo.InvariantCulture), new DoubleDouble(127.0), DoubleDouble.Parse("812563124576179134419", System.Globalization.CultureInfo.InvariantCulture)),
+        (DoubleDouble.Parse("812563124576179134546", System.Globalization.CultureInfo.InvariantCulture), DoubleDouble.Parse("812563124576179134504", System.Globalization.CultureInfo.InvariantCulture), new DoubleDouble(42.0)),
+        (DoubleDouble.Parse("812563124576179134546", System.Globalization.CultureInfo.InvariantCulture), new DoubleDouble(0.5), DoubleDouble.Parse("812563124576179134545.5", System.Globalization.CultureInfo.InvariantCulture)),
+        (DoubleDouble.Parse("812563124576179134546", System.Globalization.CultureInfo.InvariantCulture), DoubleDouble.Parse("812563124576179134546", System.Globalization.CultureInfo.InvariantCulture), new DoubleDouble(0.0)),
+    ];
+
+    [Theory]
+    [MemberData(nameof(ReferenceCases))]
+    public void SubtractionShouldProduceExpectedResults(DoubleDouble left, DoubleDouble right, DoubleDouble expected)
+    {
+        DoubleDouble result = left - right;
+
+        result.High.ShouldBe(expected.High);
+        result.Low.ShouldBe(expected.Low);
+        result.ShouldBe(expected);
+    }
+}