题面摘自 Henry E. Dudeney 公版文本;英文为古腾堡原文整理,中文为本站自译,提示、解答骨架和闲谈保留本站原创结构。
Then came forward the Tapiser, who was, of course, a maker of tapestry, and must not be confounded with a tapster, who draws and sells ale. He produced a beautiful piece of tapestry, worked in a simple chequered pattern, as shown in the diagram. "This piece of tapestry, sirs," quoth he, "hath one hundred and sixty-nine small squares, and I do desire you to tell me the manner of cutting the tapestry into three pieces that shall fit together and make one whole piece in shape of a perfect square. "Moreover, since there be divers ways of so doing, I do wish to know that way wherein two of the pieces shall together contain as much as possible of the rich fabric." It is clear that the Tapiser intended the cuts to be made along the lines dividing the squares only, and, as the material was not both sides alike, no piece may be reversed, but care must be observed that the chequered pattern matches properly.
然后塔皮瑟走上前来,他当然是挂毯制造商,不能与绘制和销售麦芽酒的酒匠混为一谈。他制作了一幅美丽的挂毯,采用简单的格子图案,如图所示。 “这块挂毯,先生们,”他说道,“有一百六十九个小方格,我确实希望您能告诉我如何将这块挂毯切割成三块,然后将它们组合在一起,形成一个完整的正方形。此外,由于这样做的方法有很多种,我确实想知道如何将其中两块挂毯一起包含尽可能多的丰富的织物。”很明显,塔皮塞尔的切割目的是为了仅沿着划分正方形的线制作,并且由于两面的材料不相同,因此不得颠倒任何部分,但必须注意方格图案是否正确匹配。
提示 1
先只保留连接关系,把多余长度和角度擦掉。
提示 2
数端点、奇偶穿越和连通块,看看有没有不变量。
提示 3
最后再把抽象关系放回原图,确认没有偷换路径。
完整解答
一笔画回到起点需要所有路口度数为偶数;若有四个奇度路口,就不可能每条路恰好走一次并回到原点。至少要改变两条端点关系,或增加一条连接两个奇度点的路,把奇度点数降到 0。
Dudeney 的趣题常把难点藏在“看起来可以试”的地方。别急着猜答案;先把图、表或状态画出来,再问哪些限制一直没有变。这也是它和 Carroll 逻辑题互补的地方:一个拆句子,一个拆结构。