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What is the smallest positive number of polyominoes P, such that

  • You can place grid aligned copies of P without any overlap; and
  • Each polyomino is adjacent to exactly 3 other polyominoes.

Polyominoes must be the same shape, but can be rotated or flipped. Two polyominoes are considered adjacent if they touch at one of their sides (not vertices).

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    $\begingroup$ See also this question. $\endgroup$ Commented Jul 24, 2023 at 6:45
  • $\begingroup$ perhaps this is duplicate then? $\endgroup$ Commented Jul 24, 2023 at 8:08
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    $\begingroup$ It has the same answer, though it is a different question. Theoretically that makes it a duplicate, but I wouldn't insist on it. $\endgroup$ Commented Jul 24, 2023 at 8:33

3 Answers 3

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The minimum number of polyominoes needed is

4

Proof:

enter image description here
This nonomino is the smallest I could find that would work.

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  • $\begingroup$ Lovely construction. Four is the definitely the minimum because each polyomino needs three neighbours. $\endgroup$ Commented Jul 24, 2023 at 2:22
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    $\begingroup$ I wonder if we can find a smaller polyomino? $\endgroup$ Commented Jul 24, 2023 at 3:14
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    $\begingroup$ @DmitryKamenetsky There is an octomino here. $\endgroup$ Commented Jul 24, 2023 at 6:46
  • $\begingroup$ oh cool! That must be optimal $\endgroup$ Commented Jul 24, 2023 at 6:59
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Here is an example of 4 octominoes, which I believe is the smallest, and also the most compact:

enter image description here

Here Bass has another octomino, as pointed by @Jaap Scherphuis. The shape of his octomino is easier, however, it takes up more space than this one, according to the number of inner white squares.

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  • $\begingroup$ Very nice! Are there solutions without any inner space? What about smaller polyominoes? $\endgroup$ Commented Jul 24, 2023 at 22:27
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    $\begingroup$ I dont think it's possible to achieve this with smaller polyominoes or without any white space. $\endgroup$ Commented Jul 25, 2023 at 4:42
  • $\begingroup$ If these were physical pieces, would they hold together without falling apart? $\endgroup$ Commented Jul 25, 2023 at 5:09
  • $\begingroup$ Less white space is possible, see my answer $\endgroup$ Commented Jul 25, 2023 at 5:20
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    $\begingroup$ If you focus on just the whitespace, you're right. But the overall shape will be bigger, as well as each polyomino. I like your thinking :) $\endgroup$ Commented Jul 25, 2023 at 7:01
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I can reduce the number of inner spaces by 3, by adding 3 cells to each piece in @Lezzup's answer:

enter image description here

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