# 带鳍三阶鱼

## 原理

因为

* 存在 3 行 3 列`RegionLineParallel[1,3]` 、\`RegionLinePerpendicular\[1,3]：
  * `RegionLineParallel[n]` 之间互相平行
  * `RegionLinePerpendicular[n]` 之间互相平行
  * `RegionLinePerpendicular[n]` 垂直于`RegionLinePerpendicular[m]`
  * `RegionLineParallel[n]` 和`RegionLinePerpendicular[m]`相交于单元格`Cell[nm]`
* 对于候选数`X`
  * 在行列`RegionLineParallel[1]` 中
    * 可填入数字`X`的单元格仅限于`Cell[11]`、`Cell[12]`、`Cell[13]`和区块 `Cells1[4,i]`
  * 在行列`RegionLineParallel[2]` 中
    * 可填入数字`X`的单元格仅限于`Cell[21]`、`Cell[22]`、`Cell[23]`
  * 在行列`RegionLineParallel[3]` 中
    * 可填入数字`X`的单元格仅限于`Cell[31]`、`Cell[32]`、`Cell[33]`

所以

* \[\[三阶鱼#删除域|三阶鱼的删除域]] 与 区块 `Cells1[4,i]` 的共同影响区域
  * 必不填入数字`X`

> 通过分析`X`是否填入区块的情况

### 技巧拓展

* \[\[三阶鱼]]：拓展鳍

> [SudokuWiki.org - Finned Swordfish](https://www.sudokuwiki.org/Finned_SwordFish)


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