diff --git a/search.ipynb b/search.ipynb
index d3dc3cca7..72300557e 100644
--- a/search.ipynb
+++ b/search.ipynb
@@ -1623,7 +1623,7 @@
     "        elif limit >= 0:\n",
     "            cutoff_occurred = True\n",
     "            limit += 1\n",
-    "            all_node_color.pop()\n",
+    "            all_node_colors.pop()\n",
     "            iterations -= 1\n",
     "            node_colors[node.state] = \"gray\"\n",
     "\n",
@@ -2162,6 +2162,8 @@
    "outputs": [],
    "source": [
     "# Heuristics for 8 Puzzle Problem\n",
+    "import math\n",
+    "\n",
     "def linear(node):\n",
     "    return sum([1 if node.state[i] != goal[i] else 0 for i in range(8)])\n",
     "\n",
@@ -2853,7 +2855,7 @@
     "        neighbor = argmax_random_tie(neighbors,\n",
     "                                     key=lambda node: problem.value(node.state))\n",
     "        if problem.value(neighbor.state) <= problem.value(current.state):\n",
-    "        \"\"\"Note that it is based on negative path cost method\"\"\"\n",    
+    "           \"\"\"Note that it is based on negative path cost method\"\"\"\n",
     "            current.state = neighbor.state\n",
     "        iterations -= 1\n",
     "        \n",
@@ -6527,7 +6529,7 @@
    "name": "python",
    "nbconvert_exporter": "python",
    "pygments_lexer": "ipython3",
-   "version": "3.6.4"
+   "version": "3.7.6"
   },
   "widgets": {
    "state": {
@@ -6561,8 +6563,17 @@
     }
    },
    "version": "1.2.0"
+  },
+  "pycharm": {
+   "stem_cell": {
+    "cell_type": "raw",
+    "source": [],
+    "metadata": {
+     "collapsed": false
+    }
+   }
   }
  },
  "nbformat": 4,
  "nbformat_minor": 1
-}
+}
\ No newline at end of file