# Smallest polyomino adjacent to 3 copies

What is the smallest polyomino P in number of cells, such that

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

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

• 4 polyominoes, all touching seems a trivial possibility even for P=1. Do you mean an infinite amount filling a 2d plane? Jul 23 at 14:09
• Sorry I don't understand how 4 monominoes can be all touching. You cant use diagonal touches, only at edges Jul 23 at 14:30
• Can we extend our polyominoes across an infinite line; can we place an infinite amount of polyominoes? Because then we could just have a $2$x$\infty$ strip of monominoes. Jul 23 at 14:42
• Can you find a polyomino that doesn't have these features?
– Bass
Jul 23 at 20:15
• Can you make the question more clear and understandable? So far your rules are rather vague, and there are many questions about them left waiting to be answered. Jul 23 at 22:01

Assuming (like Retudin's answer) that smallest P means smallest polyomino, and that placements of polyominoes have to be perfectly grid-aligned:

The optimal solution is P=2, with dominoes.

by

# With monominoes

The reason why a domino solution is optimal is that a finite solution with grid-aligned monominoes is impossible (thanks to @AxiomaticSystem for giving me the idea). Note that if the solution is allowed to be infinite, there are a lot of trivial solutions.

Why? Consider that for every monomino solution, there must be a top row of the solution (that contains at least one monomino). Then we consider one of the monominoes in the top row. Since it must be adjacent to three other monominoes and there cannot be any monominoes above it, both the left and right cells must have a monomino.

Both of these cells are still on the top row, so we can repeat the same argument ad infinitum, getting that every cell in the top row must have a monomino in it. However, this contradicts the premise of the solution being finite. Therefore, a monomino solution is impossible.

• rot13(Jung jbhyq gur gbc ebj bs n zbabzvab fbyhgvba ybbx yvxr?) Jul 23 at 21:18
• This is a brilliant answer! I didn't even know that was possible. Jul 23 at 22:48
• I am now wondering if there is a P that only requires 4 copies? Maybe I'll make another puzzle for it Jul 23 at 22:51
• @DmitryKamenetsky I'm not sure... $K_4$ appears to have some edges that cross each other; it would be akin to forcing diagonal meetings on one diagonal and then the other being blocked off. Jul 23 at 23:00
• made a new puzzle for the new question Jul 23 at 23:06

Probably not the intended answer but assuming smallest P means smallest polyomino (not smallest number of polyominos.)

P=1 is possible

• What if each polyomino has a preconstrained size: for example, monominoes have a size of 1? I have a solution for dominoes, though... Jul 23 at 14:58
• Even if only grid-aligned placements are considered to be valid, this arrangement could obviously be done using p=4. Jul 23 at 15:36
• Great answer. This is a P=4. Jul 23 at 22:49

Based on the idea of the answer of new Q Open Wid, here is a much smaller solution with dominoes I have found, using 16 dominoes:

It has a very nice symetry to it.

And because I like the idea, here the smallest I get with triominoes (both use 8 triominoes):

Edit: Just for fun: below is a figure with monominoes. Every monomino is ajacent to three others. However, as you may noticed, the grid is rolled up a bit :)

• Nice work! I had not considered ditching the 'rails' in my answer. Jul 24 at 17:47