The squares on the perimeter of the squared square or squared rectangle are the boundary squares. Those squares not on the boundary are in the interior. Those boundary squares not in corners are wall squares.
Some questions we can ask and answer;
Taking each question in turn;
For SPSRs, at least five squares must be on the boundary (in a perfect tiling the squares are all different sizes. If there were only four squares on the boundary, they would have to be the same size). For SPSSs at least seven squares must be on the boundary. Only two examples, one in order 29, side 1113 (found by Stuart Anderson, Ed Pegg Jr and Stephen Johnson) and the lowest order one in order 28 side 831 with seven squares on the boundary, found by Jasper Skinner.
In orders 9,10 SPSRs exist with only 3 squares in the interior, this is the smallest number. For SPSS, as at least 7 squares must be on the boundary, the number of interior squares is <= order - 7. The minimum number of interior squares known for SPSS is 11 in order 22. Generally for SPSS, the number of interior squares outnumbers the number of boundary squares, but in order 25, 27, 28 and 29 the opposite occurs in some rare cases. See this file for all known SPSS with the number of boundary squares greater than the number of interior squares.
Generally speaking, the largest boundary square is about twice the size of the largest interior square in a given order.
No, the largest square can be on the external wall or in the interior. Duijvestijn found SPSR with the largest corner not on the boundary in order 17. He found the first SISS with this property in order 21, and 2 lowest order SPSS with the largest corner not on the boundary in order 25. o25; 506 shows it is possible for all 4 largest squares not to be in corners.
A square of size 5 is the smallest square possible on the boundary of a perfect squared square, a result due to Gambini, the only known examples were SPSSs in order 44 and 58, both found by Gambini. Jim Williams generated complete catalogs of all SPSS up to and including order 36, and found the lowest order SPSS with a 5 square on the boundary. The Bouwkampcode is 36 1044 1044 (604,440)(164,276)(231,253,172,112)(60,328)(107,125)(209,22)(187,88)(89,18)(143)(61,27)(44,72)(38,23)(16,28)(15,8)(4,12)(1,3)(7,2)(5) * 36 : 1044U JBW 2020. In 2010 the smallest size square known on the boundary of an SPSR was an 101x91 rectangle of order 17, and an SPSR 166x143 order 22, both with a size 6. In September 2011 Brian Trial found 3 SPSRs of order 28 with '5 on the side'. The dimensions of the 3 SPSRs are; 659x506, 749x611 and 1226x979. A pdf with all 3 tilings is here. Stuart Anderson searched all SPSRs up to and including order 26, still finding 6 as the lowest size in those orders. The lowest order SPSR, order 27, with a '5 on the side' was found by Jim Williams in 2026, it completes the search and reaches the lower bound. The bouwkampcode is 27 441 390 (238,203)(115,88)(152,86)(27,61)(26,72,44)(66,20)(46)(23,38)(28,16)(8,15)(12,4)(3,1)(2,7)(5)
The smallest squares on the boundary in SPSR orders 9 to 28 are recorded as OEIS Integer Sequence A195984 and also in the following table;
| Order (SPSRs) |
Min boundary |
| 9 |
8 |
| 10 |
13 |
| 11 |
22 |
| 12 |
18 |
| 13 |
14 |
| 14 |
13 |
| 15 |
11 |
| 16 |
9 |
| 17 |
6 |
| 18 |
9 |
| 19 |
7 |
| 20 |
7 |
| 21 |
8 |
| 22 |
6 |
| 23 |
8 |
| 24 |
7 |
| 25 |
7 |
| 26 |
6 |
| 27 |
5 |
| 28 |
5 |
The bouwkampcodes (not canonical) of the 3 order 28 SPSRs found by Brian Trial;
28 659 506 204 106 71 15 7 5 12 28 62 149 2 3 8 1 4 16 23 10 34 33 9 87 35 78 98 43 357 302
28 749 611 224 99 38 15 7 5 12 28 72 249 2 3 8 1 4 16 23 44 61 27 89 125 62 63 88 387 25 362
28 1226 979 400 221 41 16 7 5 8 19 49 128 332 2 3 9 11 25 30 66 79 45 21 24 204 69 179 111 647 579
Comparing
relative
sizes of squares, the 2 smallest squares (e1 and e2) in a SPSR or SPSS cannot
appear on the boundary . Only one example, o28; 697 found by Jasper Skinner, is known of an SPSS with 3rd
smallest square (e3) on the boundary. SPSRs with the 3rd smallest
square on the boundary are known from order 13 onwards.
Relative Minima on Border - Squared Squares & Squared Rectangles
| Order |
SPSR |
SISR |
SPSS |
SISS |
| 9 |
5th |
3rd |
|
|
| 10 |
5th |
3rd |
||
| 11 |
5th |
- |
||
| 12 |
6th |
3rd |
|
|
| 13 |
3rd |
3rd |
|
3rd |
| 14 |
3rd |
5th |
||
| 15 |
3rd |
3rd |
|
|
| 16 |
3rd |
3rd |
|
|
| 17 |
3rd |
3rd |
|
3rd |
| 18 |
3rd |
3rd |
|
|
| 19 |
3rd |
3rd |
|
3rd |
| 20 |
3rd |
3rd | |
|
| 21 |
3rd | 15th |
3rd | |
| 22 |
3rd | 12th |
3rd |
According to Gambini, 9 is the smallest square possible in a corner. SPSR 32x32 order 9 is the only known SPSR or SPSS with 9 in the corner. Is this also the only possible simple perfect square tiling where 9 appears in the corner?
The 9 corner square in 33x32 is also the 5th smallest square in the tiling, a known minimum for SPSR and SPSS.
In the following table corner sizes between 10 and 14 are missing, they do not appear in any of the fully generated SPSR orders. This is quite extraordinary as there are are over 3 billion spsrs in that collection. However we do know that SPSRs with those corner sizes exist. Computer searches by Brian Trial have found examples of each. See math.stackexchange - do-perfect-squared-rectangles-with-corners-of-sizes-10-12-and-13-exist?
| Order |
Smallest corner square size = s |
| 9 (SPSRs) |
9 |
| 10 |
15 |
| 11 |
25 |
| 12 |
22 |
| 13 |
24 |
| 14 |
23 |
| 15 |
23 |
| 16 |
17 |
| 17 |
15 |
| 18 |
17 |
| 19 |
16 |
| 20 |
16 |
| 21 |
17 |
| 22 |
18 |
| 23 |
16 |
| 24 |
15 |
| 25 |
17 |
| 26 |
16 |
The smallest known corner in a SPSS, relatively speaking, is; order 27; 876. This corner square is of size 50 and is the 8th smallest square in the tiling. The smallest known absolute corner size for an SPSS is found in the two order 22 SPSS of size 110 (110 is the smallest possible size for a SPSS). Both have a square of size 18 in a corner.
The table gives SPSR largest element square sizes by order . They are all corner squares.
| Order |
Maximum
square size = s |
| 9 (SPSRs) |
36 |
| 10 |
60 |
| 11 |
105 |
| 12 |
180 |
| 13 |
329 |
| 14 |
568 |
| 15 |
996 |
| 16 |
1,733 |
| 17 |
3,068 |
| 18 |
5,315 |
| 19 |
9,400 |
| 20 |
16,439 |
| 21 |
29,019 |
| 22 |
50,550 |
| 23 |
89,064 |
| 24 |
156,430 |
| 25 |
277,848 |
| 26 |
488,520 |
If we total the number of squares of a given size in all the SPSRs of a given order, we obtain a list of ordered pairs; of the form (n, c(n)), where n is the square size , c(n) is the total of all n in the order). If we graph the ordered pairs as (x,y) coordinates, we see a curve resembling an exponential decay curve, with mod 4 oscillations on the curve.
