diff --git a/Localization/extended_kalman_filter/extended_kalman_filter_localization.ipynb b/Localization/extended_kalman_filter/extended_kalman_filter_localization.ipynb
index e1089eaf..09376a24 100644
--- a/Localization/extended_kalman_filter/extended_kalman_filter_localization.ipynb
+++ b/Localization/extended_kalman_filter/extended_kalman_filter_localization.ipynb
@@ -33,6 +33,9 @@
     "\n",
     "x, y are a 2D x-y position, $\\theta$ is orientation, and v is velocity.\n",
     "\n",
+    "In the code, xEst means the state vector. [code](https://github.com/AtsushiSakai/PythonRobotics/blob/916b4382de090de29f54538b356cef1c811aacce/Localization/extended_kalman_filter/extended_kalman_filter.py#L168)\n",
+    "\n",
+    "　\n",
     "\n",
     "The robot has a speed sensor and a gyro sensor.\n",
     "\n",
@@ -46,6 +49,8 @@
     "\n",
     "The input and observation vector includes sensor noise.\n",
     "\n",
+    "In the code, observation function generates the input and observation vector [code](https://github.com/AtsushiSakai/PythonRobotics/blob/916b4382de090de29f54538b356cef1c811aacce/Localization/extended_kalman_filter/extended_kalman_filter.py#L34-L50)\n",
+    "\n",
     "\n",
     "\n",
     "\n",