euler/ipython/EulerProblem028.ipynb

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   "source": [
    "# Euler Problem 28\n",
    "\n",
    "Starting with the number 1 and moving to the right in a clockwise direction a 5 by 5 spiral is formed as follows:\n",
    "\n",
    "~~~\n",
    "21 22 23 24 25\n",
    "20  7  8  9 10\n",
    "19  6  1  2 11\n",
    "18  5  4  3 12\n",
    "17 16 15 14 13\n",
    "~~~\n",
    "\n",
    "$1 + 3 + 5 + 7 + 9 + 13 + 17 + 21 + 25 = 101$\n",
    "\n",
    "It can be verified that the sum of the numbers on the diagonals is 101.\n",
    "\n",
    "What is the sum of the numbers on the diagonals in a 1001 by 1001 spiral formed in the same way?"
   ]
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   "source": [
    "When we know the corner of a certain spiral we can calculate it's total like $f_n = 4 c_n + 6 (n - 1)$. We then only have to update the corner value for each spiral. "
   ]
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   "source": [
    "total = 1\n",
    "current_corner = 3\n",
    "\n",
    "for n in range(3, 1002, 2):\n",
    "    total += 4 * current_corner  + 6 * (n - 1)\n",
    "    current_corner += 4 * n - 2\n",
    "\n",
    "s = total"
   ]
  },
  {
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   "execution_count": 2,
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   "outputs": [
    {
     "data": {
      "text/plain": [
       "669171001"
      ]
     },
     "execution_count": 2,
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     "output_type": "execute_result"
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   "source": [
    "assert(s == 669171001)\n",
    "s"
   ]
  },
  {
   "cell_type": "markdown",
   "metadata": {},
   "source": [
    "The only missing piece is how could we calculate the current corner value for a certain n. The series for this is as follows:\n",
    "\n",
    "$c = 1, 3, 13, 31, 57, 91, 133, 183, 241, \\dots$\n",
    "\n",
    "With some experimenting it can be seen that\n",
    "\n",
    "$c_n = (n - 1)^2 - (n - 2) = n^2 - 2n + 1 - n + 2 = n^2 - 3n + 3$.\n",
    "\n",
    "Now, we can insert $c_n$ into $f_n$ which gives as the sum of corners for the nth spiral:\n",
    "\n",
    "$f_n = 4n^2 - 12n + 12 + 6n - 6 = 4n^2 - 6n + 6$"
   ]
  },
  {
   "cell_type": "code",
   "execution_count": 3,
   "metadata": {
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   "outputs": [
    {
     "data": {
      "text/plain": [
       "669171001"
      ]
     },
     "execution_count": 3,
     "metadata": {},
     "output_type": "execute_result"
    }
   ],
   "source": [
    "s = 1 + sum([4 * n * n - 6 * n + 6 for n in range(3, 1002, 2)])\n",
    "assert(s == 669171001)\n",
    "s"
   ]
  }
 ],
 "metadata": {
  "completion_date": "Wed, 23 Aug 2017, 15:54",
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  "tags": [
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   "diagonals"
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