De_Bruijn_torus Knowpia

In combinatorial mathematics, a De Bruijn torus, named after Dutch mathematician Nicolaas Govert de Bruijn, is an array of symbols from an alphabet (often just 0 and 1) that contains every possible matrix of given dimensions $m \times n$ exactly once. It is a torus because the edges are considered wraparound for the purpose of finding matrices. Its name comes from the De Bruijn sequence, which can be considered a special case where $n = 1$ (one dimension).

One of the main open questions regarding De Bruijn tori is whether a De Bruijn torus for a particular alphabet size can be constructed for a given $m$ and $n$ . It is known that these always exist when $n = 1$ , since then we simply get the De Bruijn sequences, which always exist. It is also known that "square" tori exist whenever $m = n$ and even (for the odd case the resulting tori cannot be square).^[1]^[2]^[3]

The smallest possible binary "square" de Bruijn torus, depicted above right, denoted as $(4,4;2,2) 2$ de Bruijn torus (or simply as $B 2$ ), contains all $2\times2$ binary matrices.

B₂ edit

The (4,4;2,2) de Bruijn torus. Each 2-by-2 binary matrix can be found within it exactly once.

Apart from "translation", "inversion" (exchanging 0s and 1s) and "rotation" (by 90 degrees), no other $(4,4;2,2) 2$ de Bruijn tori are possible – this can be shown by complete inspection of all 2¹⁶ binary matrices (or subset fulfilling constrains such as equal numbers of 0s and 1s).^[4]

De Bruijn torus (8,8;3,2) containing all 64 possible 3-row × 2-column matrices exactly once, with wraparound – the bottom half is the negative of the top half

The torus can be unrolled by repeating n−1 rows and columns. All n×n submatrices without wraparound, such as the one shaded yellow, then form the complete set:

1	0	1	1	1
1	0	0	0	1
0	0	0	1	0
1	1	0	1	1
1	0	1	1	1

Larger example: B₄ edit

B₄ as a binary square matrix
The grid highlights some of the 4×4 matrices, including those of zeros and of ones at the upper margin.

An example of the next possible binary "square" de Bruijn torus, $(256,256;4,4) 2$ (abbreviated as B₄), has been explicitly constructed.^[5]

The image on the right shows an example of a $(256,256;4,4) 2$ de Bruijn torus / array, where the zeros have been encoded as white and the ones as red pixels respectively.

Binary de Bruijn tori of greater size edit

The paper in which an example of the $(256,256;4,4) 2$ de Bruijn torus was constructed contained over 10 pages of binary, despite its reduced font size, requiring three lines per row of array.

The subsequent possible binary de Bruijn torus, containing all binary $6\times6$ matrices, would have $236 = 68,719,476,736$ entries, yielding a square array of dimension $262,144\times262,144$ , denoted a $(262144,262144;6,6) 2$ de Bruijn torus or simply B₆. This could easily be stored on a computer—if printed with pixels of side 0.1 mm, such a matrix would require an area of approximately 26×26 square metres.

The object B₈, containing all binary $8\times8$ matrices and denoted $(4294967296,4294967296;8,8) 2$ , has a total of 2⁶⁴ ≈ 18.447×10¹⁸ entries: storing such a matrix would require 18.5 exabits, or 2.3 exabytes of storage. At the above scale, it would cover 429×429 square kilometres.

The following table illustrates the super-exponential growth.

n	Cells in a submatrix = n²	Number of submatrices = 2^n²	B_n side length = 2^(n²/2)
2	4	16	4
4	16	65 536	256
6	36	68 719 476 736	262 144
8	64	~1.84×10¹⁹	~4.29×10⁹
10	100	~1.27×10³⁰	~1.13×10¹⁵
12	144	~2.23×10⁴³	~4.72×10²¹
14	196	~1.00×10⁵⁹	~3.17×10²⁹
16	256	~1.16×10⁷⁷	~3.40×10³⁸
18	324	~3.42×10⁹⁷	~5.85×10⁴⁸
20	400	~2.60×10¹²⁰	~1.61×10⁶⁰

References edit

^ Fan, C. T.; Fan, S. M.; Ma, S. L.; Siu, M. K. (1985). "On de Bruijn arrays". Ars Combinatoria A. 19: 205–213.
^ Chung, F.; Diaconis, P.; Graham, R. (1992). "Universal cycles for combinatorial structures". Discrete Mathematics. 110 (1): 43–59. doi:10.1016/0012-365x(92)90699-g.
^ Jackson, Brad; Stevens, Brett; Hurlbert, Glenn (Sep 2009). "Research problems on Gray codes and universal cycles". Discrete Mathematics. 309 (17): 5341–5348. doi:10.1016/j.disc.2009.04.002.
^ Eggen, Bernd R. (1990). "The Binatorix B2". Private communication.
^ Shiu, Wai-Chee (1997). "Decoding de Bruijn arrays constructed by the FFMS method". Ars Combinatoria. 47 (17): 33–48.