Merge pull request #1537 from pybamm-team/double-negation

double negation case

pybamm/expression_tree/binary_operators.py

@@ -1104,7 +1104,10 @@ def simplified_multiplication(left, right):
                 return (left * r_left) + (left * r_right)
 
     # Negation simplifications
-    if isinstance(left, pybamm.Negate) and right.is_constant():
+    if isinstance(left, pybamm.Negate) and isinstance(right, pybamm.Negate):
+        # Double negation cancels out
+        return left.orphans[0] * right.orphans[0]
+    elif isinstance(left, pybamm.Negate) and right.is_constant():
         # Simplify (-a) * b to a * (-b) if (-b) is constant
         return left.orphans[0] * (-right)
     elif isinstance(right, pybamm.Negate) and left.is_constant():
@@ -1193,6 +1196,9 @@ def simplified_division(left, right):
                 return l_left * new_right
 
     # Negation simplifications
+    if isinstance(left, pybamm.Negate) and isinstance(right, pybamm.Negate):
+        # Double negation cancels out
+        return left.orphans[0] / right.orphans[0]
     elif isinstance(left, pybamm.Negate) and right.is_constant():
         # Simplify (-a) / b to a / (-b) if (-b) is constant
         return left.orphans[0] / (-right)

pybamm/expression_tree/unary_operators.py

@@ -1116,8 +1116,15 @@ def div(symbol):
     # Divergence commutes with Negate operator
     if isinstance(symbol, pybamm.Negate):
         return -div(symbol.orphans[0])
-    else:
-        return Divergence(symbol)
+    elif isinstance(symbol, (pybamm.Multiplication, pybamm.Division)):
+        left, right = symbol.orphans
+        if isinstance(left, pybamm.Negate):
+            return -div(symbol._binary_new_copy(left.orphans[0], right))
+        # elif isinstance(right, pybamm.Negate):
+        #     return -div(symbol._binary_new_copy(left, right.orphans[0]))
+
+    # Last resort
+    return Divergence(symbol)
 
 
 def laplacian(symbol):

tests/unit/test_expression_tree/test_binary_operators.py

@@ -501,6 +501,7 @@ def test_binary_simplifications(self):
         self.assertEqual((c * -1).id, (-c).id)
         self.assertEqual((-1 * c).id, (-c).id)
         # multiplication with a negation
+        self.assertEqual((-c * -f).id, (c * f).id)
         self.assertEqual((-c * 4).id, (c * -4).id)
         self.assertEqual((4 * -c).id, (-4 * c).id)
         # multiplication with broadcasts
@@ -532,6 +533,7 @@ def test_binary_simplifications(self):
         self.assertEqual((c / c).id, pybamm.Scalar(1).id)
         self.assertEqual((broad2 / broad2).id, broad1.id)
         # division with a negation
+        self.assertEqual((-c / -f).id, (c / f).id)
         self.assertEqual((-c / 4).id, (c / -4).id)
         self.assertEqual((4 / -c).id, (-4 / c).id)
         # division with broadcasts

tests/unit/test_expression_tree/test_unary_operators.py

@@ -190,6 +190,12 @@ def test_div(self):
         div = pybamm.div(-pybamm.Gradient(a))
         self.assertEqual(div.id, (-pybamm.Divergence(pybamm.Gradient(a))).id)
 
+        div = pybamm.div(-a * pybamm.Gradient(a))
+        self.assertEqual(div.id, (-pybamm.Divergence(a * pybamm.Gradient(a))).id)
+
+        # div = pybamm.div(a * -pybamm.Gradient(a))
+        # self.assertEqual(div.id, (-pybamm.Divergence(a * pybamm.Gradient(a))).id)
+
     def test_integral(self):
         # space integral
         a = pybamm.Symbol("a", domain=["negative electrode"])