improved test for stackVariable

Author: Serrof

hipparchus-core/src/main/java/org/hipparchus/analysis/differentiation/FieldGradient.java

@@ -880,7 +880,7 @@ public FieldGradient<T> linearCombination(final double a1, final FieldGradient<T
     /**
      * Add an independent variable to the Taylor expansion.
      * @return object with one more variable
-     * @since 13.0
+     * @since 4.0
      */
     public FieldGradient<T> stackVariable() {
         final T[] gradient = MathArrays.buildArray(getValueField(), this.getFreeParameters() + 1);

hipparchus-core/src/main/java/org/hipparchus/analysis/differentiation/Gradient.java

@@ -715,7 +715,7 @@ public Gradient linearCombination(final double a1, final Gradient b1,
     /**
      * Add an independent variable to the Taylor expansion.
      * @return object with one more variable
-     * @since 13.0
+     * @since 4.0
      */
     public Gradient stackVariable() {
         final double[] gradient = new double[this.getFreeParameters() + 1];

hipparchus-core/src/test/java/org/hipparchus/analysis/differentiation/FieldGradientTest.java

@@ -33,6 +33,7 @@ void testStackVariable() {
         // THEN
         Assertions.assertEquals(gradient.getValue(), gradientWithMoreVariable.getValue());
         Assertions.assertEquals(gradient.getFreeParameters() + 1, gradientWithMoreVariable.getFreeParameters());
+        Assertions.assertEquals(0., gradientWithMoreVariable.getGradient[gradient.getFreeParameters().getReal()])
         Assertions.assertArrayEquals(gradient.getGradient(), Arrays.copyOfRange(gradientWithMoreVariable.getGradient(),
                 0, gradient.getFreeParameters()));
     }

hipparchus-core/src/test/java/org/hipparchus/analysis/differentiation/GradientTest.java

@@ -103,6 +103,7 @@ void testStackVariable() {
         // THEN
         Assertions.assertEquals(gradient.getValue(), gradientWithMoreVariable.getValue());
         Assertions.assertEquals(gradient.getFreeParameters() + 1, gradientWithMoreVariable.getFreeParameters());
+        Assertions.assertEquals(0., gradientWithMoreVariable.getGradient[gradient.getFreeParameters()])
         Assertions.assertArrayEquals(gradient.getGradient(), Arrays.copyOfRange(gradientWithMoreVariable.getGradient(),
                 0, gradient.getFreeParameters()));
     }