add copyright notice

notebooks/problem0056.ipynb

@@ -46,6 +46,16 @@
    "source": [
     "max(digit_sum(a^b) for a in range(1, 100) for b in range(1, 100))"
    ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "d6f7d0af",
+   "metadata": {},
+   "source": [
+    "#### Copyright (C) 2025 filifa\n",
+    "\n",
+    "This work is licensed under the [Creative Commons Attribution-ShareAlike 4.0 International license](https://creativecommons.org/licenses/by-sa/4.0/) and the [BSD Zero Clause license](https://spdx.org/licenses/0BSD.html)."
+   ]
   }
  ],
  "metadata": {