Problem of the Month (May 1999)

The surround number of a polyomino is the number of different ways that the polyomino can be surrounded by non-overlapping copies of the same shape. A polyomino is considered surrounded if every unit edge is shared completely by a neighboring polyomino. Reflections of polyominoes are not allowed.

For example, the surround number of a cross is 2, the tiling below and its mirror image:

Which positive integers are the surround numbers of some shape? In particular, is 3 a surround number? What are the surround numbers of the small polyominoes? What is the surround number of an m x n rectangle? For what other families of polyominoes can we compute surround numbers?

Brendan Owen, Andrew Bayly, and Joe DeVincentis proved that every positive integer is the surround number of some polyomino.

Owen found a general formula for the surround number of an m×n rectangle. It's pretty complicated. An n×n square has surround number 2n2-1. For n ≥ 2, an n×1 rectangle has surround number (n4+22n3+105n2-56n-8) / 4. For n ≥ 3, an n×2 rectangle has surround number (n4+32n3+344n2+768n+400) / 16 if n is even and (n4+32n3+278n2+656n+361) / 16 if n is odd.

Here are some polyominoes with surround number 3, and the people who found them:

Polyominoes with Surround Number 3

PolyominoAuthor
Erich Friedman
Brendan Owen
Andrew Bayly
Joe DeVincentis

Here are the surround numbers of some small polyominoes:

Small Polyominoes

PolyominoSurround Number
1
123
7555
361
7
161
13355
2794
778

Here are the smallest polyominoes with a given surround number. These are all due to Brendan Owen.

Smallest Polyominoes with Small Surround Numbers

Surround NumberPolyominoArea
07
11
25
39
48
59
69
74
88
97
109
Surround NumberPolyominoArea
119
129
139
148
159
166
179
188
198
209

This sequence of values of the smallest area polyomino to have surround number n: 7, 1, 5, 9, 8, 9, 9, 4, 8, 7, 9, 9, 9, 9, 8, 9, 6, 9, 8, 8, 9, 8, 8, 7, 9, 8, 7, 8, 8, 9, 8, 8, 9, 9, 8, 9, 9, 9, 9, 8, 9, 6, 8, 9, 10, 9, 9, 8, 6, 7, 8, 8, 8, 9, 8, 9, 9, 9, 10, 9, 9, 8, 9, 9, 9, 9, 9, 9, 9, 8, 9, 9, 7, 8, 8, 9, 9, 7, 8, 7, 9, 9, 9, 9, 9, 9, 8, 9, 9, 9, 9, 9, 9, 9, 8, 9, 9, 8, 9, 9, 9, . . . is now sequence A047875 of the Encyclopedia of Integer Sequences.

If you can extend any of these results, please e-mail me. Click here to go back to Math Magic. Last updated 3/5/02.