public SecondDerivativeFunction(DualNumberFunction parent, int variableIndex1, int variableIndex2)
 {
     this.parent = parent;
     this.variableIndex1 = variableIndex1;
     this.variableIndex2 = variableIndex2;
 }
 public FirstDerivativeFunction(DualNumberFunction parent, int variableIndex)
 {
     this.parent = parent;
     this.variableIndex = variableIndex;
 }