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回文である数式

Posted at

概要

文字列アルゴリズム advent calendar 2016 の21日目の記事にある

142+382×567=765×283+241

と同様の回文を紹介します。

ウォームアップ

  • 12 x 12 x 441 = 12 x 12 x 21 x 21 = 144 x 21 x 21.
  • 13 x 13 x 961 = 13 x 13 x 31 x 31 = 169 x 31 x 31.

これらの回文では式の値(それぞれ63504, 162409)は回文ではありません。

式の値も回文である例

  • 12 x 231 = 2772 = 132 x 21,
  • 12 x 2331 = 27972 = 1332 x 21,
  • 12 x 23331 = 279972 = 13332 x 21,
  • ...

  • 12 x 2121 = 25452 = 1212 x 21,
  • 12 x 21021 = 252252 = 12012 x 21,
  • 12 x 210021 = 2520252 = 120012 x 21,
  • ...

  • 12 x 23331 = 132 x 2121 = 279972 = 1212 x 231 = 13332 x 21
  • 12 x 2333331 = 1332 x 21021 = 27999972 = 12012 x 2331 = 133332 x 21,
  • 12 x 233333331 = 13332 x 210021 = 2799999972 = 120012 x 23331 = 133333332 x 21,
  • ...

上記の回文はいずれも繰り上がりを含まず多項式として扱えます. (例えば 231 は 2x^2+3x+1 とみなす.)
そうではない次のような例もあります

96 x 933 x 2599 = 23 x 96 x 113 x 933 = 232787232 = 339 x 311 x 69 x 32 = 9952 x 339 x 69.

Open?

  • 最後の例の一般化
  • 冒頭に引用したような足し算を含むタイプなど, もっとかっこいい回文を探す

引用した回文以外は自分で見つけましたが, 既出なら引用先を教えて頂けるとありがたいです。

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