Subsets of the Hexominoes (Slight Return)

 A while ago (four years!) I wrote a post about the subsets of the hexominoes that result if you separate them into the piece containing a 2x2 block and those that don't. I have no idea what I was smoking when I wrote that, because I make some wild claims regarding the parity imbalance properties of these sets, which a reader comment by 'dshamlin' kindly points out.

Fig. 1: The set of 27 hexominoes that don't contain a 2x2 square. Those with imbalanced parity are shown in red/yellow.

I somehow had difficulty counting to eight because I stated that seven of the set of 27 non-2x2 hexominoes had unbalanced parity and that therefore the whole set was unbalanced and couldn't tile rectangles. No idea which one I missed (or how the fact that removing an unbalanced set from the unbalanced full set of hexominoes and getting yet another unbalanced set didn't strike me as weird) but the fact is these pieces absolutely can tile rectangles. A 9x18 is shown below, a 6x27 is possible, and since 162 factorises quite nicely there are probably other shapes (sets of 3 identical rectangles..?) out there to be solved too.

Here's a solution for the 9x18. It was just found with Peter Esser's 'mops' solver for quickness, proof of concept type thing. Though I've got a good mind to roll my sleeves up, crack out the set of plastic hexominoes, discard the 2x2-block ones and have a go at manually solving some shapes with this set now that I know that a lot can be done with them.

In fact that'd be as good an excuse as any to start posting here with a bit more regularity - part of why posting here died off is that I felt like I'd done a lot of what I could do with the polyforms I had table space to play with. More constructions with hexominoes, heptominoes, octominoes just felt like variations on a theme that's been done to death here. Enneominoes I suppose was the logical extension of that but the logistics of solving anything other than the few rectangles that fit on my table in my room are a bit of a pain. Dekominoes onwards are out. Big polyiamonds are too hard, big polyhexes are too big, and aside from some dabbling in one-sided polyforms or a few constructions with woefully wonky homemade polycubes, I felt like I'd exhausted nearly all the novelty. But evidently there's a lot I overlooked first time round (or wasn't aware even were possibilities, what with not exactly being the brightest knife in the drawer sometimes).

Fig. 3: A Figure 8.

Here's another one using those 27 'tricky' hexominoes - again, computer solved because lazy. In fact just looking at these pieces terrifies me, the idea of hand-solving. Which bits do you hold onto for the late-game? Yeah, there's a few L's and S's , and some of the U-pentomino descendents look like they might fit nicely in corners, but I get the feeling it's going to be a case of discovering a whole new range of techniques here...

Solving Technique: 'Piece Substitution'

By sheer chance when solving a hexomino thing a few days ago I was presented with a really nice clear example of a technique that I'm sure I always just referred to in blog posts and things as 'piece substitution' but never actually ever bothered to clarify. So here it is. Consider yonder picture:

The two pieces in red to the side are the two I'm left with, and the hole remaining just won't accommodate them in any way short of physically snapping the pieces apart. The best we can do is getting the more irregular piece in there in the obvious place, leaving a longer thinner 'L' shaped gap than we're capable of filling. Like this.

The trick here is to look at the two pieces in light blue, one of which is the long skinny 'L' piece we need. Notice that we can do this:

which uses up our unusable P-shaped piece, and at the same time frees up out long piece, allowing us to fill the other hole and complete the puzzle.

Of course, there's no guarantee that it'll fall into place as nicely as that. Sometimes it's two pairs of pieces that can make the same shape that need to be swapped, or sometimes it's even uglier, like a chain of substitutions that free up one particular piece then use that piece to free up another. But it's a viable technique surprisingly often given how much of an utter fluke it looks.

Subsets of the Hexominoes

Like hexominoes, but are overwhelmed by the sheer number (35) (!) of them? Well, have I got just the puzzle for you. It's these:

The set consisting of the eight hexominoes that contain a 2x2 block is... less than fantastic. Sure, at first glance it looks interesting, a manageable set of cooperative pieces, but scratch the surface and you hit the snag that seems to ruin everything in the world of tetrominoes and hexominoes (how the octominoes avoid it is a mystery to me) - parity. The set has an imbalance of ±2, so kiss goodbye the hopes of being able to make any rectangle with an even edge length, and many other symmetrical constructions. To alleviate this problem somewhat, I've added to the set a single monomino which allows a 7x7 square to be filled. It's natural to want to solve it with the monomino in an aesthetically pleasing place - the centre or a corner - but due to the aforementioned parity constraints none of these is possible. In fact, the only places the monomino can go are the black squares on the diagram below, which means there are effectively four different positions for it excluding rotation and reflection.

All of these are solvable - holes at positions 1 through 4 have 11, 22, 11 and 5 solutions respectively. But the ones with the notch at the centre of an edge are undeniably the best.

If you take the pieces off-road, away from the confines of the tray, there are other shapes you can wring out of them too with or without the supplemental monomino. A 5x10 with two corners on the short side removed is possible with the eight hexominoes, and there are (if FlatPoly2 is to be believed) 47 ways of doing this.

Fig. 1: Here's one of them

There are also a few configurations of the 5x10 with two holes down its long line of symmetry which have solutions, and I'm beginning to think that maybe I should have made the wooden puzzle a 10x5 frame with two monominoes and it would have been more interesting.

Again, numbers are excluding rotations and reflections. And these numbers are of unique solutions, it doesn't guarantee interesting solutions. Several may be related to each other in that reflecting a symmetrical subpart of one solution made of two or three hexominoes could lead to another. Still counts. I always felt a little bit cheated by this as a kid, looking at the 2,339 distinct solutions of the pentominoes in a 6x10 rectangle. But I grudgingly came to accept that cases like these were in fact solutions in their own right.

Fig. 3: Like this. This would have made nine-year-old me's blood boil. (Not really.)

In fact, that example solution before is even more egregious an example than I'd first realised. The two pieces at the top (the fish and the one that looks like a backwards δ) can also be flipped over, giving four possible solutions (two for each of the configurations of the orange part). And then the entire non-orange section of the image can be reflected and rotated, giving a total of 2x2x4 = 16 solutions for the price of one.

The Remaining 27 Hexominoes

EDIT 14/11/26: This whole section below here is wildly wrong: somehow I counted seven parity-unbalanced pieces but there's eight so everything I say about rectangles being impossible is all complete cack. Thanks to commenter 'dshamlin' for bringing this to my attention. I'll leave the original post here for posterity's sake, but just take it with a massive grain of salt, and see here for an exploration of what this set of pieces can actually do.

Perversely, taking a parity-unbalanced subset away from the hexominoes doesn't leave a nice balanced set remaining. Of the 27 hexominoes that don't include a 2x2 sub-square, seven of them have 4-2 balancing when checkerboard-coloured, which means we've still got all the same problems as with the full set. In addition, we've took out eight pieces which are some of the easiest to work with, meaning that the resulting set is going to be a real challenge to work with, at least if we're solving by hand.

Fig. 4: Here's the set crammed into the first shape I could think of, solved by computer because I got lazy. It's like 28°C here right now, it's lucky I can be arsed to even write a blog post.

The other subgroup of the hexominoes that suggests itself is the set of 15 symmetrical hexominoes (including both rotational and mirror symmetry). However, these pieces are just so uncooperative I've found it pretty much impossible to solve anything pretty or interesting with them, and I've tried on and off for the past couple of months.

Picking this back up after a long break

Man, I've missed this so much. Only one post so far this year and it was ages ago. Basically, stuff just got kind of busy, a surprise change of job in February/March took up a big chunk of my time, and with my attention divided between the blog and the shiny new website I ended up not contributing a great deal to either. But~! Enough excuses. A few nights ago - just for the hell of it, just for old times' sake - I dug out the heptominoes and just had a crack at solving something with them. Just a 29x29 rectangle with a central 9x9 hole (and some other unit holes). Knowing full well it wouldn't be something interesting enough to write to the blog or the site about.

And I remembered why I started doing this in the first place.

Spring 2019 - when this blog started) - was, for me, not exactly the most fun of times. And I think in those evenings of stumbling into the world of polyforms, retreading the steps people like David Bird and Michael Keller had made way before me, it became a sort of meditative thing. When I was knee deep in a hexomino or heptomino thing all I was able to focus on were the pieces at hand (and occasionally the Andrew W.K. I had blasting on the iPod) and it was a welcome break from everything else going on. Kept me sane (or at least, kept me from getting any less sane than I already was...)

And I think that's what I need now.

Fig. 1: I finally made good on that promise to buy a proper camera instead of just using the one on my phone. Problem now is, I don't know how to use it (and I scale down the photos for the site anyway to save space) so they still look just as bad.

It's not like I've got a lack of things to write about on the blog, anyway. In the past 6 months or so I've been getting things made (generally laser cut) like no-one's business, though these generally lie in the more 'out there' realms of polyforms - sets where the base shape is something weird like a domino sliced in half diagonally, or assembly puzzles of mathematically incomplete polycube sets. Think Soma cube, and its many variations and relatives. Enough to keep churning posts out on here anyhow.

Here's a picture of some actual polyominoes, as way of apology for turning this place into LiveJournal up there before:

Fig. 2: Four-colouring these adds an interesting challenge.

Oh, and I made this too.

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It's the nine enneominoes that have either the 2x4 octomino or the 3x3 with a missing corner as a subset. There's one solution (excluding rotations and reflections) for getting them into that 9x9 box, and I'll leave it as a puzzle for the reader (if indeed there are any).

I made this stupidly small - the pieces are at a scale of 5mm/edge meaning the entire thing including tray is a mere 65mm wide. I was trialling how small I could feasibly make pieces and have them still be nice to play with. Because I have plans for a certain large set of pieces. And evidently 5mm/edge is just a tad too small. Might have to do 8mm.

Some Assorted Solutions

Polyform-related shenanigans have been on the back burner somewhat recently, mainly because I'm in the process of moving house and don't have table space right now to spread out even a full set of hexominoes, let alone a larger set. So this post is going to be a collection of solutions I found several weeks ago but didn't write about at the time for whatever reason.

First up, continuing with the last post's theme of long skinny rectangles we've got the heptominoes in a 9x85 rectangle with nine holes. I think I found this with the intention of including it in that post, but then I found the 5 high rectangle which was way more interesting and in the interest of not making that post a mile long I took this one out.

In February some time I put together another variant of the well-known side 20 diamond found originally by David Bird - same outer shape, different hole configuration. Nothing particularly groundbreaking to look at, but the solving process was interesting. The jagged outer edges use up pretty much every piece with any stairstep-type edges, leaving a glut of pieces with 2-cell protrusions for the middle. Which is less than ideal.

And finally, I found a few more fun things with the hexominoes, in what I call the 15x15-15 challenge because I'm terrible with names like that. The idea is, make a 15x15 square with 15 holes using the hexominoes. The challenge is to make the 15 holes look nice, given that they can't retain the symmetry of the outer square. There's some examples here (from back in 2019) and a few more just below, from back in March.

A silicon ship looking configuration.

Side note: For whatever reason the word 'quincunx' makes me giggle like an idiot.

Oh yeah. One last thing that I almost didn't include since it's a shameful chapter indeed in the history of polyominoes. I had attempted a heptomino construction with the crenelated edges similar to that top hexomino one. And I made the classic mistake of doing the math for it late one night when I was too tired to be trusted with simple tasks like adding a few numbers up. This was the result:

Yep. For whatever reason I'd thought that each edge had one more protrusion than it did which threw my area calculation off by 4. And because it was late and I was just itching to get my solve on, I didn't even think to double-check everything. I just never learn...

2021! (a.k.a. A Fun Little Challenge with Heptominoes and Hexominoes)

Happy new year! Going to kick off 2021 with a lazy low effort post.

In an older post I found two hexomino rectangles which had twelve holes, each in the shape of a different pentomino. Or it could be thought of as a combined pentomino/hexomino rectangle with the added condition that none of the pentominoes touch each other or the edge of the rectangle.

The topic was brought up on the 'Puzzle Fun' Facebook group, and the gauntlet was thrown down - was the same thing possible for heptominoes with hexomino-shaped holes? I had a go and found the following:

Fig. 1: 22x44. Separating the hexominoes and heptominoes out when dismantling the finished construction was not a fun job.

There's two monomino holes in there, one has to be there to placate the harbour heptomino, the second is there because otherwise the combined area (7*108 + 6*35 + 1 = 967) would be a prime number. And the hexominoes are at a higher concentration in the bottom half of the rectangle. I wasn't sure how much space I had when I started off constructing this so I packed them in really tightly at first, then when I got about three-quarters of the way through I realised I had like 4 hexominoes left so they're a tad sparser up there.

the inevitable next step will be the octomino/heptomino equivalent of this. I'm putting off starting it but I'd give it a month tops before I cave and begin trying to solve it.

Squares

This has bound to have been done before, it seems too obvious a thing not to. But rediscovery or not I'm going to write about it at length anyway and nobody can stop me.

Imagine this: squares, right, but made out of polyominoes. Gripping stuff, eh?

Pentominoes

With tetrominoes and below you can't really do a lot, for all the usual reasons. Tetrominoes have got parity imbalance, triominoes and dominoes are just tiny sets you can't do a lot with, and while I suppose you can make a 1x1 square out of the set of monominoes (all one of them) it's not the most interesting thing in the world so we'll jump ahead to where it starts to get interesting.

One set of pentominoes can pack a 5x5 square and a 6x6 square simultaneously, if you don't mind an off-centre hole in the 6x6 to bring its total area down to 35. Shown above is one way of doing this, but there are at least 2 more, not counting rotations and reflections as distinct solution. Sadly there is no combination of squares without holes that the pentominoes can fill, just because 60 can't be partitioned into square numbers divisible by 5.

Hexominoes

This lot are frustrating due to the usual parity constraints but nevertheless we'll give it a go. Square sizes permitted are 6² = 36 and 12² = 144 for solid squares, and 5² = 24+1, 7² = 48+1, 11² = 120+1, 13² = 168+1 if you want to allow a square with one solitary central hole. Which feels like bending the rules but alleviates parity issues somewhat, and will be needed for heptominoes and above where we've got pieces with holes whether we like it or not.

Making 210 with a combination of the above squares is our next problem. Each of the above squares would contain an even number of hexominoes, meaning that no combination of them would ever contain exactly 35 hexominoes. Damn.

One avenue to explore is to allow duplication of one of the hexominoes, bringing the set size up to 36 and total area to 216. This can be broken into squares, the most obvious partition being six 6x6 squares. This is... difficult. I've tried it by hand, I've tried running various software to find a solution, no luck so far. All I know is that for this to work the duplicated hexomino must be one of the ones with 2-4 parity imbalance. And that it's possible to solve six 6x6s for the set of hexominoes plus triominoes. So if solutions do exist, they look to be pretty few and far between.

But there are other ways of making 216 from the above numbers. 6² + 6² + 12² works, as does 7² + 13² with a central hole in each. The duplicated hexomino in each case is shown in a different colour (again, the duplicate must be one of the 11 pieces with unbalanced parity.)

6² + 6² + 12² = 216

 

7² + 13² = 216 + 2

As a general rule, the more squares and the smaller the squares are, the harder it is to find a solution.

5² + 5² + 7² + 11² = 216 + 4

Another fun challenge would be to place the two duplicate pieces in a symmetrical or otherwise aesthetically pleasing way. I've just been letting pure chance dictate which piece is duplicated and where it ends up, i.e. solving the 35 unique pieces and hoping the 6 remaining squares are all joined together. It works but the solutions aren't always the prettiest.

Heptominoes

This is where it starts getting fun. For the first time we've got a real choice of how many squares we can do and what size they are. For unholey squares the sizes possible are 7², 14² and 21² (28² > 757 so we don't need to worry about that) and for squares with a centre hole (which we're gonna need for the harbour heptomino) the available sizes are 13² and 15². Actually, a 6² or 8² would be possible but the hole would be off-centre so I didn't consider these. We can afford to be picky here.)

I didn't do an exhaustive list of what was possible, I just found a bunch of squares whose area added up to 756 and jumped right in. The first solution I found was the picture below.

Three 7x7s, a 13x13 and 21x21 with heptominoes.

I did the little squares first because they're the most restrictive in terms of which pieces can be used then did the great big square at the very end. I then realised that instead of having a 21x21 square, I could create four 7x7s, two 14x14s and a 13x13 which would yield a little more challenging of a solve.

 

'A little more challenging' is putting it lightly.

For a start, it took me two attempts. Admittedly this was down to my own stupidity - on the first attempt I'd done the four small squares first then moved onto the larger three, only to discover near the end that one of my 7x7 squares was actually a 6x7 and I had one leftover piece too many. The second attempt (the one pictured above) just took ages to do. Same solve order, but when I got to the last square (the bottom right 14x14) I was left with some really difficult uncooperative pieces and completing it took probably about six hours spread out over the space of the weekend.

The obvious next step here is this: One 14x14 is equivalent to four 7x7s. So It should be possible to solve a full set of heptominoes into the following:

• Eight 7x7s, a 13x13 and a 14x14.
• Twelve 7x7s and a 13x13.
  

Whether either of these is possible (or feasible solving by hand) is another matter entirely. I noticed with the construction with four 7x7s I was running out of pieces that would fit comfortably in corners or along edges by the end of the last square, and introducing even more edge is only going to make that worse. I might tackle those other possibilities at some point soon. But that's a pretty big 'might'.

Little stopgap post while I think of something decent to write about

It's been a little while since I last posted anything on here. Here's a nice 4-way symmetrical heptomino construction, does that make it better?

Fig. 1: Those corners are surprisingly restricting of which heptominoes can make them.

 

In the past I used to crack open the box of polyform stuff in evenings as a way of relaxing and calming down, but recently I stumbled upon this monstrosity which seems to have the opposite effect on me:

Fig. 2: Eww gross.

Called the 'sawblade' in my little notebook where I sketch out possible constructable shapes, and although its just a pure hexomino puzzle it's way harder than it has any right to be. I found the above sort of near-miss early on, which fit all the pieces but had an asymmetrical clump for a central hole which just looks wrong. I mean, even the 'proper' solution has a 2-fold symmetrical hole in the middle of an otherwise 4-fold construction so it's never going to be perfect, but this was just too imperfect to live with.

Fig. 3: Better (marginally...)

It took a further three days of trying on and off, just whenever I had half an hour or so spare until I finally stumbled on a solution that had the central hole looking some way presentable. The real nightmare pieces in solving this were the long straight bits, the I-hexomino and the various pieces with a 5xn bounding box. And the big L-shape piece. Usually these can be sat against the flat walls of rectangular constructions but not in this case... Learning to use these up early on seemed to be the key to cracking this one.

I'm toying with the idea of laser-cutting a fresh new set of hexominoes (again...) this time using transparent acrylic so I can see the boundaries between pieces. It's tempting to make a tray for them shaped like the above pattern, especially since it's approximately square so the entire tray would be relatively compact. I could maybe have the central heptomino 'hole' in a contrasting colour too. The only drawback of all this would be that every time I wanted to tidy the pieces away into the tray I'd have to go through the ordeal of solving it.

 

 29th? Yesss! didn't miss a month!

A Fun Little Challenge with Pentominoes and Hexominoes

 I've probably seen this done somewhere else but I can't think where or else I'd drop a link in and let them explain it better than I can. Basically, the idea is to create a rectangle (or another shape, I won't judge) out of the pentominoes and hexominoes together - 270 units total so you've got a few choices here. But, there are a few restrictions you can place on the positioning of the pentominoes.

For the easiest option, forbid the pentominoes from touching each other (either by sharing an edge or touching point-to-point.) Or if you want to ramp up the challenge a bit, prohibit the pentominoes from touching the edge of the rectangle too. This is essentially a hexomino construction with 12 pentomino-shaped holes in it, if you want to think about it that way. Below are pictures for the 15x18 and 10x27 cases, but there are several other aspect ratios possible, the thinner the rectangles become the harder I imagine it'll be to keep the pentominoes away from one another.

Fig. 1: The 15x18 solution in a colour scheme that would have Ikea's lawyers frothing at the mouth

Fig. 2: A 10x27 solution

Little mini post: 4x53 with Hexominoes, Revisited

 Yonks ago I wrote a post about 4xn constructions with the hexominoes, which ended with suggesting a 4x53 with symmetrical placement of holes should be possible. Well, I found one:

Amazingly, the entire solution took all of three minutes by hand. I must have just been insanely lucky and hit on a solution on practically the first try, because I was prepared for this to take quite a while.

I guess they're not technically 'holes' when they're on the very edge of the construction but it looks nice so I don't mind really. And for an additional bonus, it's three-colourable and has no 4-way crossroads where four pieces meet at a point. That wasn't even my intention while solving it, it's just how it turned out. Rrrresult!

Miscellaneous Solutions That Didn't Deserve Their Own Posts

Sometimes polyomino-related things have a decent story behind them (or, failing that, a really boring story that can be stretched out to blog-post proportions.) But sometimes they don't. Today it's a selection of the latter; digitised solution pictures that were just sat around cluttering up the folder named 'BLOG STUFF' on my desktop, to tide me over while I write up some actually half-decent posts.

Rhombus with Hexominoes

Difficulty level: Mild (approx. two chilies out of five)

I've seen this, or variations of it, done before so it's not really particularly groundbreaking as solutions go (although really, are any of them that groundbreaking?) Including it here because it was a hard-won battle - I kept building the edges wrong, accidentally adding in steps of size 1 or 3 then not realising until right near the end when I was left with an internal hole whose size wasn't a multiple of six.

Heptomino Rectangle with 21 Holes

Difficulty level: Breakin' a Sweat

Solved the middle first, since the closely-packed holes are quite restricting on what pieces can even go there. But then the rest just solved like a normal heptomino construction and I've banged on about those at length in other posts so it wasn't really worth doing another one.

And the harbour heptomino doesn't need to be in the very centre of solutions like these, but it just feels wrong any other way.

5x45 rounded Rectangle with 11 Holes

Difficulty level: Real Tears

This was an utter nightmare, combining two of my worst fears into one shape: 5xn with hexominoes is always an ordeal, and adding that row of holes just pushes it over the edge into the kind of territory where it's actually frustrating and unpleasant to solve. You can see by the way the eight pieces with 2x2 blocks in them are scattered all over the shop that my usual solving technique only got me so far before I was left to fend for myself, desperately applying trial-and-error for several hours of my life I'll never get back.

Recommendation: FlatPoly2 can probably crack this one in under 10 seconds, just do that instead.

Stay tuned, next time I might actually have something a bit more substantial.

The cruel irony of all this is I'm awful at Tetris

I've been playing with a set of hexominoes a lot recently. It seems the bottom of the barrel wasn't as thoroughly scraped as I'd previously thought.
It turns out there are a family of shapes that approximate hexagons with angles of 90 and 135 degrees that are just as versatile as rectangles, if a little less pleasing to the eye (especially if you're not too fussy about them having a few holes in 'em.) These also have an added bonus: the diagonal edges make these a slightly more challenging solution than squares and rectangles.

Again, odd x odd bounding boxes seem to work best with this, because an even width or height would mean a line of symmetry bisecting the entire shape into two congruent 110-cell sub-shapes, which would mean the overall shape has balanced checkerboard parity. So with the aid of a hideous and confusing spreadsheet I knocked together, I found the following set of solvable problems:

3x73 - 9 holes
Yeah no. I think we can generally rule out the possibility of any purely 3xn constructions with hexominoes. I roughly outlined my reasoning here (but that's not a rigorous mathematical proof so it could well be wildly incorrect...)

5x54 - 3 holes
Pros: At this width solving these shapes feels not much different from solving a normal rectangle.
Cons: Solving a width 5 rectangle isn't exactly a walk in the park.

7x35 - 11 holes
At this point there's a sort of 'choose your own difficulty' option built in - with the difficulty levels being hard, harder and hardest, naturally. Two immediate options for arranging the 11 internal holes are having them as one big 11-omino, or as a line of 11 individual monominoes. The long 11-omino option creates two stretches of treacherous 3xn space which takes a bit of doing:

Whereas the individual holes introduce all manner of feisty challenges.

This one's left as an exercise for the reader. I spent a little while trying to solve it and got fairly close, but not close enough. I think I gave it like 45 minutes or so and wound up with like 6 pieces remaining a few times. But they were never nice pieces.

9x29 - 11 holes

This one was tough, although in hindsight this could be because of a complete lack of solving strategy. The obvious (looking back now) thing to do would have been to solve the ends first, then either the top half or the bottom half of the middle (if we think of the row of holes as a dividing line), trying to leave a fairly convex-looking endgame to be filled in last with the 2x2-block-y pieces. Instead I just went at it the way a nine-year old would his first all-you-can-eat buffet, and I paid for it dearly by spending maybe close to two hours (!) getting those last few bits in.
You know you've utterly stuffed it when you're down to the last five pieces and one of them is the stair-step hexomino (as it's known in Conway's Game of Life terminology).

11x25 - 5 holes
This is the start of the sweet spot, so to speak. Wide enough so that the central holes don't get in the way too much, but not so wide that there's miles of diagonal edge to contend with.

Of course, the fact that there's only 5 holes maybe makes this a little easier too. You could just as easily do this with the holes arranged vertically. Or with one bigger hole shaped like a pentomino of your choice.

13x23 - 5 holes

Can be done with the five holes the other way on. (So can the 11-height one but I didn't think of it at the time.)

I think there is a 15xn possible but it's going to have like 17 holes or something, and I think there's a point where unless you can arrange them in a particularly pleasing way it just gets silly. So I'll skip over this and go straight to...

17x21 - 3 holes and 19x21 - 9 holes
No solution images here, for the simple reason that I just plain can't be arsed. If you want them that badly, acquire a set of hexominoes and have a go at finding them yourself. More fun than looking at a picture of someone else's solution, guaranteed. Or feed them into a solver program, they'll eat these for breakfast.

And finally, 21x21 - 11 holes
This is just a rotated square at this point. And I've seen a solution for this on the internet somewhere, with the 11 holes as the straight 11-omino dead centre. As far as I can remember it was illustrating something to do with parity imbalance, and this was the example where the difference between black and white squares is exactly ±22, the maximum permissible. I assume that the restrictions that places on parity-imbalanced pieces means it's a head-bendingly difficult solution.

[EDIT: The solution I saw somewhere was here, on page 9 of Chessics, issue 28]

(The above solution was found using a computer search. Because it sounded like it might be difficult to do, and I'm always one to back down from a difficult challenge when one presents itself.)

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After playing with octominoes for a bit, returning to the trusty ol' heptomino set is a strange feeling indeed. All the pieces are so tiny and simple in comparison, and suddenly 108 feels like such a reasonable, manageable number.
I made this, which has the maximum possible symmetry for a heptomino pattern.

Fig. 1: 28x28 with 28 holes.

This was a surprisingly painless solve too (although anything would feel that way after octominoes) - the last pieces all fitted into place on practically the first attempt and the whole solve couldn't have taken much more than half an hour. (And now there's an awful idea: speed-solving polyominoes!)

And here's some assorted hexomino things from 2019 that never made it onto the site because they weren't particularly challenging or interesting.

But these are the lengths you gotta go to when you've been too busy recently to do any interesting polyomino stuff but don't want to completely abandon the blog.

Attempting 4xn Constructions with Hexominoes

4xn is about as tight as you can squeeze hexominoes.

3xn looks like it's pretty much impossible. Consider the blue pieces in the following image:

All four of these need to be in the final construction (and will only fit horizontally) but between them they create six 3-cell deep wells at the edge of the construction. And there's only five hexominoes (the red ones) that could fill those gaps. The I-hexomino could theoretically fill two 3-cell deep wells, but I think in every possible case the space between the two blue hexominoes either end of it would be less than 6 and therefore unfillable. Unless we're going for constructions with holes permitted, in which case that would be fine after all. But exceedingly difficult.

Then, check out these green cases that create two adjacent 2-cell deep wells. In filling one of them, either you have to use one of the red pieces from above, or you use a piece that has a 2-cell extension, which would then cover the square marked by the red 'X' and create a new 3 (or more) cell deep well.

I know this isn't a rigorous, mathematically watertight proof but it's enough of a deterrent to stop me spending ages looking for 3xn solutions.


So, back to 4-cell high...
Similar to how it is with pentominoes (the narrower the rectangle, the fewer solutions there are), finding 4xn rectangles with hexominoes has proven to be surprisingly challenging. The few search programs I know how to use don't seem to like really narrow rectangles very much either, which left me doubtful I'd be able to do much better by hand. (Although in hindsight it's more likely I just don't know how to use the programs as well as I think I do, or how to set them up so that they search efficiently.)

A few months back I'd found the solution below using the combined set of hexominoes and pentominoes. It's not really what I'm aiming for though; the addition of the smaller pentominoes makes this about a hundred times easier, and I was able to place the holes symmetrically as a result.

On 31/12/19 I had another crack at this, aiming for a 4x53 grid using just the hexominoes, but with no constraints on where the two holes would be. Just proving that a 4xn solution exists would be enough for the first step; making it all pretty could come later.

After far too long (about an hour, maybe? It's hard to say because I tend to lose track of time when doing things like this) I found a solution. This one:

It's butt ugly though; not only are the two holes not placed in any kind of order but one is on the edge of the rectangle too, which just doesn't look right to me. It's not really a hole now, is it? It's just some weird notch out of the side of the puzzle. Oh well, it's a start.

Side note, that 'T' piece near the right-hand end was the absolute worst piece to place. Had I used it up right near the start it might not have been such a pain in the arse, but somehow it escaped my attention until there wasn't a lot of long skinny pieces left that worked well with it. In order to place it vertically rather than horizontally, it needed to have one of the holes either side of it too, since it partitions the rectangle into two parts, both of odd size.

But, it's a proof of concept at least. A symmetrical 4x53 rectangle seems way more possible now than it did before. (Edit 06/09/2020: I found one!)