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   "source": [
    "Every third fibonacci number is even. Since $F_n = F_{n-3} * 4 + F_{n-6}$, we initialy calculate $F_3$ and $F_6$ then $F_9 = F_6 * 4 + F_3$, $F_{12} = F_9 * 4 + F_6$ etc..."
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   "source": [
    "def Fn(Fn_3, Fn_6):\n",
    "    return Fn_3 * 4 + Fn_6\n",
    "Fn_6 = 0\n",
    "Fn_3 = 2\n",
    "sum = Fn_6 + Fn_3\n",
    "while True:\n",
    "    f = Fn(Fn_3, Fn_6)\n",
    "    if f > 4_000_000:\n",
    "        break\n",
    "    sum += f\n",
    "    Fn_6 = Fn_3\n",
    "    Fn_3 = f"
   ]
  }
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  "title": "Even Fibonacci Numbers"
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