Fix DerivativeStructure.pow for 0 argument and power less than 1.

src/main/java/org/apache/commons/math4/analysis/differentiation/DSCompiler.java

@@ -893,6 +893,12 @@ public class DSCompiler {
             return;
         }
 
+        if (operand[operandOffset] == 0) {
+            // special case, 0^p = 0 for all p
+            Arrays.fill(result, resultOffset, resultOffset + getSize(), 0);
+            return;
+        }
+
         // create the function value and derivatives
         // [x^p, px^(p-1), p(p-1)x^(p-2), ... ]
         double[] function = new double[1 + order];

src/test/java/org/apache/commons/math4/analysis/differentiation/DerivativeStructureTest.java

@@ -345,6 +345,18 @@ public class DerivativeStructureTest extends ExtendedFieldElementAbstractTest<De
                     Assert.assertEquals(0.0, d, Precision.SAFE_MIN);
                 }
             }
+
+            // 0^p with p smaller than 1.0
+            DerivativeStructure u = new DerivativeStructure(3, maxOrder, 1, -0.0).pow(0.25);
+            for (int i0 = 0; i0 <= maxOrder; ++i0) {
+                for (int i1 = 0; i1 <= maxOrder; ++i1) {
+                    for (int i2 = 0; i2 <= maxOrder; ++i2) {
+                        if (i0 + i1 + i2 <= maxOrder) {
+                            Assert.assertEquals(0.0, u.getPartialDerivative(i0, i1, i2), 1.0e-10);
+                        }
+                    }
+                }
+            }
         }
 
     }