From 6a93d0b8090153bf3f30215cc4000b688426f9a3 Mon Sep 17 00:00:00 2001
From: Stephen Crowley <stephencrowley214@gmail.com>
Date: Mon, 2 Dec 2024 11:05:13 -0600
Subject: [PATCH] =?UTF-8?q?public=20void=20testDerivativeOfLogarithm()=20?=
 =?UTF-8?q?=20=20{=20=20=20=20=20var=20x=20=3D=20RealFunction.parse("?=
 =?UTF-8?q?=E2=88=82ln(x)/=E2=88=82x");=20=20=20=20=20assertEquals("1/x",?=
 =?UTF-8?q?=20x.rootNode.toString());=20=20=20}=20https://github.com/crowl?=
 =?UTF-8?q?ogic/arb4j/issues/253?=
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---
 .../java/arb/expressions/nodes/unary/FunctionNode.java     | 4 ++++
 src/test/java/arb/expressions/ExpressionTest.java          | 7 +++++++
 2 files changed, 11 insertions(+)

diff --git a/src/main/java/arb/expressions/nodes/unary/FunctionNode.java b/src/main/java/arb/expressions/nodes/unary/FunctionNode.java
index af6864ecc..d84fc07cb 100644
--- a/src/main/java/arb/expressions/nodes/unary/FunctionNode.java
+++ b/src/main/java/arb/expressions/nodes/unary/FunctionNode.java
@@ -26,6 +26,7 @@
 import arb.expressions.Expression;
 import arb.expressions.FunctionMapping;
 import arb.expressions.Parser;
+import arb.expressions.nodes.LiteralConstantNode;
 import arb.expressions.nodes.Node;
 import arb.expressions.nodes.VariableNode;
 import arb.functions.Function;
@@ -473,6 +474,9 @@ private Node<D, R, F> differentiateBuiltinFunction()
                                 expression).neg(); // derivative of cos is -sin
     case "exp":
       return this; // derivative of exp is exp
+    case "log":
+      return new LiteralConstantNode<>(expression,
+                                       "1").div(arg);
     // Add other built-in function derivatives
     default:
       throw new UnsupportedOperationException("Derivative not implemented for function: " + functionName);
diff --git a/src/test/java/arb/expressions/ExpressionTest.java b/src/test/java/arb/expressions/ExpressionTest.java
index eb5ffc6f9..753a72a9e 100644
--- a/src/test/java/arb/expressions/ExpressionTest.java
+++ b/src/test/java/arb/expressions/ExpressionTest.java
@@ -31,6 +31,13 @@
 public class ExpressionTest extends
                             TestCase
 {
+  public void testDerivativeOfLogarithm()
+  {
+    var x = RealFunction.parse("∂ln(x)/∂x");
+    assertEquals("1/x", x.rootNode.toString());
+  }
+
+  
   public void testDerivativeOfExponentialFunction()
   {
     var x = RealFunction.parse("∂exp(x)/∂x");