added the 'optimized' version of Multiply(scale) from Double.SparseVector to the other three SparseVectors

src/Numerics/LinearAlgebra/Complex/SparseVector.cs

@@ -706,6 +706,28 @@ namespace MathNet.Numerics.LinearAlgebra.Complex
             return result;
         }
 
+        /// <summary>
+        /// Multiplies a scalar to each element of the vector.
+        /// </summary>
+        /// <param name="scalar">The scalar to multiply.</param>
+        /// <returns>A new vector that is the multiplication of the vector and the scalar.</returns>
+        public override Vector<Complex> Multiply(Complex scalar)
+        {
+            if (scalar == Complex.One)
+            {
+                return Clone();
+            }
+
+            if (scalar == Complex.Zero)
+            {
+                return new SparseVector(Count);
+            }
+
+            var copy = new SparseVector(this);
+            Control.LinearAlgebraProvider.ScaleArray(scalar, copy._nonZeroValues, copy._nonZeroValues);
+            return copy;
+        }
+
         /// <summary>
         /// Multiplies a scalar to each element of the vector and stores the result in the result vector.
         /// </summary>

src/Numerics/LinearAlgebra/Complex32/SparseVector.cs

@@ -736,6 +736,28 @@ namespace MathNet.Numerics.LinearAlgebra.Complex32
             return result;
         }
 
+        /// <summary>
+        /// Multiplies a scalar to each element of the vector.
+        /// </summary>
+        /// <param name="scalar">The scalar to multiply.</param>
+        /// <returns>A new vector that is the multiplication of the vector and the scalar.</returns>
+        public override Vector<Complex32> Multiply(Complex32 scalar)
+        {
+            if (scalar == Complex32.One)
+            {
+                return Clone();
+            }
+
+            if (scalar == Complex32.Zero)
+            {
+                return new SparseVector(Count);
+            }
+
+            var copy = new SparseVector(this);
+            Control.LinearAlgebraProvider.ScaleArray(scalar, copy._nonZeroValues, copy._nonZeroValues);
+            return copy;
+        }
+
         /// <summary>
         /// Multiplies a scalar to each element of the vector and stores the result in the result vector.
         /// </summary>

src/Numerics/LinearAlgebra/Single/SparseVector.cs

@@ -688,6 +688,28 @@ namespace MathNet.Numerics.LinearAlgebra.Single
             return result;
         }
 
+        /// <summary>
+        /// Multiplies a scalar to each element of the vector.
+        /// </summary>
+        /// <param name="scalar">The scalar to multiply.</param>
+        /// <returns>A new vector that is the multiplication of the vector and the scalar.</returns>
+        public override Vector<float> Multiply(float scalar)
+        {
+            if (scalar == 1.0f)
+            {
+                return Clone();
+            }
+
+            if (scalar == 0f)
+            {
+                return new SparseVector(Count);
+            }
+
+            var copy = new SparseVector(this);
+            Control.LinearAlgebraProvider.ScaleArray(scalar, copy._nonZeroValues, copy._nonZeroValues);
+            return copy;
+        }
+
         /// <summary>
         /// Multiplies a scalar to each element of the vector and stores the result in the result vector.
         /// </summary>