adding atan

CHANGELOG.md

@@ -7,6 +7,7 @@
 -   Added `ProcessedSymbolicVariable` class, which can handle symbolic variables (i.e. variables for which the inputs are symbolic) ([#940](https://github.com/pybamm-team/PyBaMM/pull/940))
 -   Made `QuickPlot` compatible with Google Colab ([#935](https://github.com/pybamm-team/PyBaMM/pull/935))
 -   Added `BasicFull` model for lead-acid ([#932](https://github.com/pybamm-team/PyBaMM/pull/932))
+-   Added 'arctan' function  ([#973]https://github.com/pybamm-team/PyBaMM/pull/973)
 
 ## Optimizations
 

pybamm/expression_tree/functions.py

@@ -430,3 +430,21 @@ def _function_diff(self, children, idx):
 def tanh(child):
     " Returns hyperbolic tan function of child. "
     return pybamm.simplify_if_constant(Tanh(child), keep_domains=True)
+
+
+class Arctan(SpecificFunction):
+    """ Arctan function """
+
+    def __init__(self, child):
+        super().__init__(np.arctan, child)
+
+    def _function_diff(self, children, idx):
+        """ See :meth:`pybamm.Function._function_diff()`. """
+        return 1 / (children[0] ** 2 + 1)
+
+
+def arctan(child):
+    " Returns hyperbolic tan function of child. "
+    return pybamm.simplify_if_constant(Arctan(child), keep_domains=True)
+
+

tests/unit/test_expression_tree/test_functions.py

@@ -142,6 +142,22 @@ def test_arcsinh(self):
             places=5,
         )
 
+    def test_arctan(self):
+        a = pybamm.InputParameter("a")
+        fun = pybamm.arctan(a)
+        self.assertIsInstance(fun, pybamm.Arctan)
+        self.assertEqual(fun.evaluate(inputs={"a": 3}), np.arctan(3))
+        h = 0.0000001
+        self.assertAlmostEqual(
+            fun.diff(a).evaluate(inputs={"a": 3}),
+            (
+                pybamm.arctan(pybamm.Scalar(3 + h)).evaluate()
+                - fun.evaluate(inputs={"a": 3})
+            )
+            / h,
+            places=5,
+        )
+
     def test_cos(self):
         a = pybamm.InputParameter("a")
         fun = pybamm.cos(a)