diff --git a/lecture_003/pmpp.ipynb b/lecture_003/pmpp.ipynb
index 69c2f77..b361cce 100644
--- a/lecture_003/pmpp.ipynb
+++ b/lecture_003/pmpp.ipynb
@@ -1407,6 +1407,16 @@
     "}'''"
    ]
   },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "id": "c0075560",
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "cpp_src = \"torch::Tensor rgb_to_grayscale(torch::Tensor input);\""
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,