Implemented DoTransposeThisAndMultiply for matrices on Double.

src/Numerics/LinearAlgebra/Double/DenseMatrix.cs

@@ -461,7 +461,7 @@ namespace MathNet.Numerics.LinearAlgebra.Double
             else
             {
                 Control.LinearAlgebraProvider.MatrixMultiplyWithUpdate(
-                    Algorithms.LinearAlgebra.Transpose.DontTranspose,
+                    Algorithms.LinearAlgebra.Transpose.Transpose,
                     Algorithms.LinearAlgebra.Transpose.DontTranspose,
                     1.0,
                     Data,
@@ -482,7 +482,28 @@ namespace MathNet.Numerics.LinearAlgebra.Double
         /// <param name="result">The result of the multiplication.</param>
         protected override void DoTransposeThisAndMultiply(Matrix<double> other, Matrix<double> result)
         {
-            throw new NotImplementedException();
+            var denseOther = other as DenseMatrix;
+            var denseResult = result as DenseMatrix;
+
+            if (denseOther == null || denseResult == null)
+            {
+                base.DoTransposeThisAndMultiply(other, result);
+            }
+            else
+            {
+                Control.LinearAlgebraProvider.MatrixMultiplyWithUpdate(
+                                  Algorithms.LinearAlgebra.Transpose.Transpose,
+                                  Algorithms.LinearAlgebra.Transpose.DontTranspose,
+                                  1.0,
+                                  Data,
+                                  RowCount,
+                                  ColumnCount,
+                                  denseOther.Data,
+                                  denseOther.RowCount,
+                                  denseOther.ColumnCount,
+                                  0.0,
+                                  denseResult.Data);
+            }
         }
 
         /// <summary>

src/Numerics/LinearAlgebra/Double/Matrix.cs

@@ -269,6 +269,28 @@ namespace MathNet.Numerics.LinearAlgebra.Double
             }
         }
 
+        /// <summary>
+        /// Multiplies the transpose of this matrix with another matrix and places the results into the result matrix.
+        /// </summary>
+        /// <param name="other">The matrix to multiply with.</param>
+        /// <param name="result">The result of the multiplication.</param>
+        protected override void DoTransposeThisAndMultiply(Matrix<double> other, Matrix<double> result)
+        {
+            for (var j = 0; j < other.ColumnCount; j++)
+            {
+                for (var i = 0; i < ColumnCount; i++)
+                {
+                    var s = 0.0;
+                    for (var l = 0; l < RowCount; l++)
+                    {
+                        s += At(l, i) * other.At(l, j);
+                    }
+
+                    result.At(i, j, s);
+                }
+            }
+        }
+
         /// <summary>
         /// Multiplies the transpose of this matrix with a vector and places the results into the result vector.
         /// </summary>