move notebooks to subdir

notebooks/problem0020.ipynb

@@ -0,0 +1,65 @@
+{
+ "cells": [
+  {
+   "cell_type": "markdown",
+   "id": "c1d94bd8",
+   "metadata": {},
+   "source": [
+    "# [Factorial Digit Sum](https://projecteuler.net/problem=20)\n",
+    "\n",
+    "Piece of cake for SageMath."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": 1,
+   "id": "894e7489",
+   "metadata": {},
+   "outputs": [
+    {
+     "data": {
+      "text/plain": [
+       "648"
+      ]
+     },
+     "execution_count": 1,
+     "metadata": {},
+     "output_type": "execute_result"
+    }
+   ],
+   "source": [
+    "sum(map(int, str(factorial(100))))"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "id": "6dab9653",
+   "metadata": {},
+   "source": [
+    "## Relevant sequences\n",
+    "* Sum of digits of $n!$: [A004152](https://oeis.org/A004152)"
+   ]
+  }
+ ],
+ "metadata": {
+  "kernelspec": {
+   "display_name": "SageMath 9.5",
+   "language": "sage",
+   "name": "sagemath"
+  },
+  "language_info": {
+   "codemirror_mode": {
+    "name": "ipython",
+    "version": 3
+   },
+   "file_extension": ".py",
+   "mimetype": "text/x-python",
+   "name": "python",
+   "nbconvert_exporter": "python",
+   "pygments_lexer": "ipython3",
+   "version": "3.11.2"
+  }
+ },
+ "nbformat": 4,
+ "nbformat_minor": 5
+}