Introduction We want to find 2D patterns of ones (dots)and zeros (blanks) for which the planar autocorrelation function has minimum out-of-phase values. These “minimum agreement” patterns may be viewed as a generalization of the 1D “ruler problems”. For example minimizes in a array with four dots.

Related problems We consider patterns of dots in rectangular grid under the following requirements: For horizontal and vertical noncyclic shifting, the shifted pattern will overlap with the original one in at most one dot

Problem Let denote the maximal number of dots in an array. Known values of : For every there exists a construction, for , such that (it is not known if holds for all ). On the other hand, it is proven that there exists a constant such that (Erdos-Turan).

Related problems We consider patterns of dots in rectangular grid under the following requirements: For horizontal and vertical noncyclic shifting, the shifted pattern will overlap with the original one in at most one dot The patterns will be of size and will have one dot in every row and column

Problem These patterns exist for infinitely many values of , nevertheless, there is no construction for general value of . There are (permutations). We denote to be the sequence of It seems very probable that and therefore we will not rely on random constructions of arrays.

Requirements We consider patterns of dots in rectangular grid under the following requirements: For horizontal and vertical noncyclic shifting, the shifted pattern will overlap with the original one in at most one dot The patterns will be of size and will have one dot in every column (rows are not restricted)

Sonar In the sonar application our array is a sequence of distinct frequencies in consecutive time slots . Vertical shift happens due to the Doppler effect (velocity) and horizontal shift happens due to the time elapsed (range). In this case our objective is to maximize the number columns for a given .

Sonar In the sonar application our array is a sequence of distinct frequencies in consecutive time slots . Vertical shift happens due to the Doppler effect (velocity) and horizontal shift happens due to the time elapsed (range). In this case our objective is to maximize the number columns for a given . Let be maximal m for which the out-of-phase agreement is at most dots.

Sonar for Let be maximal m for which the out-of-phase agreement is at most dots.

Related problem We consider patterns of dots in rectangular grid under the following requirements: The patterns will be of size and will have one dot in every column (rows are not restricted) For only horizontal noncyclic shifting, the shifted pattern will overlap with the original one in at most one dot

Radar Radar application might not require Doppler measurement and will not care for vertical shifts. We would still try to maximize the number columns for a given . “max columns”

Difference triangle algorithm This is an algorithm to check the validity of a array with one dot per column. This is the corresponding triangle to the array . To check the validity of the array, make sure that the rows in the triangle, have no repeated signed differences.

Connection to 1D A connection between 1D “ruler problems”, and some of the 2D min-agreement patterns can be established through the following:

Another problem One more related problem is to find a pattern with the minimal number of dots in a rectangle, such that no additional dot can be placed without causing a repeat pair.

Constructions The objective: For each construct a permutation matrix such that, difference vectors are all distinct as vectors. Such matrices are called Costas Arrays.

Welch construction Theorem 1: Let be a primitive root modulo the prime . Then the permutation matrix with iff ,

Welch construction Theorem 1: Let be a primitive root modulo the prime . Then the permutation matrix with iff , Proof: Consider the two pairs where , and assume equality of the difference vectors , and then Hence this pattern is a Costas Array

Welch construction Theorem 1: Let be a primitive root modulo the prime . Then the permutation matrix with iff , Example: For the Costas Array is

Cont’ Lemma 1: If an Costas Array has 1 in any of its four corners, the corresponding row and column can be removed to obtain an Costas Array. Proof: Any violation in the reduced pattern would have been a violation I the original pattern.

Cont’ Corollary 1.1: We can obtain a Costas Array, from the Welch construction. Proof: Since , the original array of degree has a 1 in which is a corner, and so by Lemma 1, the array can be reduced to degree .

Cont’ Corollary 1.2: If 2 is a primitive root modulo then, we can obtain a Costas Array, from the Welch construction. Proof: After removing the 1 at position , the 1 at position becomes a corner, and can also be removed by Lemma 1.

Cont’ Corollary 1.3: Every cyclic permutation of the rows of a Costas Array in the Welch construction, is again a Costas Array. Proof: Let be a primitive root modulo and let be any fixed positive integer. Then the permutation matrix with iff , is a Costas Array of degree and the successive values of give the successive cyclic permutation of the rows in the Welch construction. Note: Costas Array property is preserved under the group of dihedral symmetries of the square.

Lempel construction Theorem 2: Let be a primitive element in the field , for then the symmetric permutation matrix with iff , , is a Costas Array.

Lempel construction Theorem 2: Let be a primitive element in the field , for then the symmetric permutation matrix with iff , , is a Costas Array. Proof: If we may write . Consider the two pairs and assume equality of the difference vectors and then

Lempel construction and since , the above requires . Hence this pattern is a Costas Array.

Lempel construction and since , the above requires . Hence this pattern is a Costas Array. Example: Let and let be a root of the polynomial . The Costas Array is

Cont’ Corollary 2.1: If 2 is a primitive root of modulo , then a symmetric Costas Array can be constructed. Proof: Taking and , we get that which is a corner, and so by applying Lemma 1, we reduce the pattern to degree

Golomb construction Theorem 3: Let be primitive elements in the field , for then the permutation matrix with iff , , is a Costas Array. (generalization of Theorem 2)

Golomb construction Theorem 3: Let be primitive elements in the field , for then the permutation matrix with iff , , is a Costas Array. (generalization of Theorem 2) Proof: If we may write . The rest is exactly as in the proof of Theorem 2.

Cont’ Example: Let and let be a root of the polynomial , and let . The Costas Array is

Cont’ Example: Let and let be a root of the polynomial , and let . The Costas Array is Additional Corollaries: Corollary 2.2 (Taylor): if in satisfies then by removing we get Costas Array of degree . Corollary 3.1: if then by removing we get Costas Array of degree . Corollary 3.2: if and then and so by removing we get Costas Array of degree .

Cont’ Corollary 3.3: if and by the arithmetic of exponents and thus by removing , we will get a pattern of degree . Corollary 3.4: if and then necessarily and thus by removing , we will get a pattern of degree .

Special properties Periodicity Repeating Costas Array in both directions over the entire plain, gives a doubly periodic checkerboard pattern. It is proven that for all , there does not exist a doubly periodic pattern. Repeating the degree pattern, which we obtain from the Welch construction, will get us singly periodic pattern.

Special properties Nonattacking Queens For there is no known Costas Array consisting of nonattacking queens. Regarding semi-Queens, we have an infinite supply of patterns, from the Lempel construction, which are valid due to the symmetry of the Lempel construction.

Special properties Nonattacking Queens For there is no known Costas Array consisting of nonattacking queens. Regarding semi-Queens, we have an infinite supply of patterns, from the Lempel construction, which are valid due to the symmetry of the Lempel construction. Semi-Queen – attacks its row, column, and only the diagonal parallel to the main diagonal.

Special properties Nonattacking Queens For there is no known Costas Array consisting of nonattacking queens. Regarding semi-Queens, we have an infinite supply of patterns, from the Lempel construction, which are valid due to the symmetry of the construction. If is a power of an odd prime there will be exactly one dot on the main diagonal, and if is a power of 2, there will be none.

Special properties Shearing Distinctness of differences is preserved while applying non-singular linear transformation. For example: using linear transformation on theLempel construction with will in fact produce a rotation of itself.

Special Properties Shearing Distinctness of differences is preserved while applying non-singular linear transformation. For example: using linear transformation on theLempel construction with will in fact produce a rotation of itself.

Shearing cont’ Only few Costas Arrays are shearable by into another Costas Array. For this to happen, the array must have one dot in each of consecutive lines parallel to the main diagonal – those become the columns after the shearing. The rows remain rows, and the columns become lines orthogonal to the main diagonal. Almost all shearable arrays go through a cycle of four different patterns via shearing alternately by and

Shearing cont’ The following array goes through a cycle of twelve patterns:

Special properties Honeycomb Arrays Shear-compression by will convert the square cells into hexagonal cells. When dealing with Costas Arrays with nonattacking semi-queens we can delete the unoccupied diagonal lines, and apply shear-compression to get a ”honeycomb array”. The semi-queens become (still) nonattacking bee-Rooks.

Honeycomb Arrays

Honeycomb Arrays Some definitions: Bee-Dukes – is a piece which can move to any one of the six adjacent cells (on hexagonal boards). The distance between two cells in the hexagonal Lee metric – is the minimal number of bee-Duke moves needed to go from one cell to another. Lee sphere of radius – consists of a center cell, together with all the cells at distance . Note: All known honeycomb arrays with nonattacking bee-Rooks are in fact a Lee sphere, but it is not yet proven to always be the case.

Honeycomb Arrays Some related problems: The cuban primes of Cunningham show up when we count the number of cells on a Lee sphere of radius . This number is always of the form , and is often a prime. The zero sum arrays of Bennett and Potts arrive at the problem of counting , which is the number of configurations of nonattacking bee-Rooks on a honeycomb which is a Lee sphere of radius . Let be the number of configurations inequivalent under the dihedral group of symmetries of the hexagon.

Honeycomb Arrays Another counting problem is to count all the honeycomb arrays with the requirement that all differences be distinct between the nonattacking bee-Rooks on a honeycomb board of radius . Let be the total number of honeycomb arrays of radius Let be the number of honeycomb arrays of radius inequivalent under the dihedral group of symmetries of the hexagon.

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