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Regular complex Hadamard matrices of order 7,8

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Abstract

A complex Hadamard matrix H∈MN(C)H\in M_N(\mathbb C) is called regular when the scalar products between its rows ∑kHikHˉjk=0\sum_kH_{ik}\bar{H}_{jk}=0 decompose as trivially vanishing sums of roots of unity, rotated by scalars. We review the general theory of such matrices, including the known classification at N=6N=6, and then we work out classification results at N=7N=7 and N=8N=8.

Introduction

A complex Hadamard matrix is regular when the entrywise quotient of any 2 rows splits into prime cycles: rotated copies of the full set of ppth roots of unity, for primes pp. Fixing the cycles gives integer linear congruences in the entry phases. We prove that their solution sets are finitely many torsion-translated tori, so every regular matrix is an affine deformation of a Butson matrix. We then classify the regular matrices at orders 7 and 8.

Equivalence consists of row and column permutations and unit scalings; conjugation and transposition are separate operations. At order 7 the quotient types are 7, 5 + 2, and 3 + 2 + 2. There are 6 affine 3-row families and 45 isolated 3-row classes. Completing them gives precisely the Fourier matrix F7F_7 and the 2 families below, with ω=e2πi/3\omega=e^{2\pi i/3} and q∈Tq\in\mathbb{T}.

P7q=(−qqω1ω1ωq−qω11ωωωω−ω1ωω1111−1ωωωω1ωω−q‾ωq‾ω11ωωωq‾ω−q‾ω1ωω1ω11−1)R7q=(1111111111ω−1−1ω21−ω2ω2ω2q−qω1ω−11qω2−qω2ω21−1−ω−1−qq−ω21ω2ωqωq−qω−q1ω2ω−qωqω−qq)P_7^q=\begin{pmatrix} -q & q & \omega& 1 & \omega& 1 & \omega\\ q & -q & \omega& 1 & 1 & \omega& \omega\\ \omega& \omega& -\omega& 1 & \omega& \omega& 1\\ 1 & 1 & 1 & -1 & \omega& \omega& \omega\\ \omega& 1 & \omega& \omega& -\overline{q}\omega& \overline{q}\omega& 1\\ 1 & \omega& \omega& \omega& \overline{q}\omega& -\overline{q}\omega& 1\\ \omega& \omega& 1 & \omega& 1 & 1 & -1 \end{pmatrix} \qquad R_7^q=\begin{pmatrix} 1 & 1 & 1 & 1 & 1 & 1 & 1\\ 1 & 1 & 1 & \omega& -1 & -1 & \omega^2\\ 1 & -\omega^2 & \omega^2 & \omega^2 & q & -q & \omega\\ 1 & \omega& -1 & 1 & q\omega^2 & -q\omega^2 & \omega^2\\ 1 & -1 & -\omega& -1 & -q & q & -\omega^2\\ 1 & \omega^2 & \omega& q\omega& q & -q\omega& -q\\ 1 & \omega^2 & \omega& -q\omega& q\omega& -q & q \end{pmatrix}

The family R7qR_7^q is a monomially normalized conjugate of P7qP_7^q. The 2 displayed branches are retained because conjugation is not part of equivalence.

At order 8 the possible cycle lengths in a row quotient are A=2+2+2+2A=2+2+2+2, B=3+3+2B=3+3+2, and C=5+3C=5+3. Here 2 denotes an opposite pair, 3 a regular triangle, and 5 a regular pentagon, each rotated independently. The table lists all regular matrices up to equivalence. Its first group has only quotients of types A,BA,B; in the second group, type CC occurs for at least one pair of rows.

Row quotientsMatricesDimension
Only A,BA,BF8,5a,F8,5b,F8,5cF_{8,5a}, F_{8,5b}, F_{8,5c}5 each
F8,4,F8,4tF_{8,4}, F_{8,4}^{t}4 each
F8,2,F8,2t,K8,2F_{8,2}, F_{8,2}^{t}, K_{8,2}2 each
M8,1,I8,12M_{8,1}, I_{8,12}1, 0
CC occursK8,30,L8,30,L8,30tK_{8,30}, L_{8,30}, L_{8,30}^{t}0

Table 1.

The 9 continuous families are displayed below. All parameters range independently over the unit circle; the transpose families are written out explicitly.

F8,5a(a,b,c,d,e)=(111111111−1−bdbd−bdbd1−11−1c−bdadbda−d−11c11−1−1−1−1111−1cbda−d−bdad−11c1−1bd−bdbd−bd1−111ceaeb−e−aeb−1−1c11c−e−aebeaeb−1−1c)F_{8,5a}(a,b,c,d,e)= \begin{pmatrix} 1&1&1&1&1&1&1&1\\ 1&-1&-bd&bd&-bd&bd&1&-1\\ 1&-\frac{1}{c}&-\frac{bd}{a}&d&\frac{bd}{a}&-d&-1&\frac{1}{c}\\ 1&1&-1&-1&-1&-1&1&1\\ 1&-\frac{1}{c}&\frac{bd}{a}&-d&-\frac{bd}{a}&d&-1&\frac{1}{c}\\ 1&-1&bd&-bd&bd&-bd&1&-1\\ 1&\frac{1}{c}&e&\frac{ae}{b}&-e&-\frac{ae}{b}&-1&-\frac{1}{c}\\ 1&\frac{1}{c}&-e&-\frac{ae}{b}&e&\frac{ae}{b}&-1&-\frac{1}{c} \end{pmatrix}

References

  1. [4]T. Banica, J. Bichon and J.-M. Schlenker, Representations of quantum permutation algebras, J. Funct. Anal. 257 (2009), 2864–2910.arxiv.org/abs/0901.2331
  2. [6]T. Banica, D. Özteke and L. Pittau, Isolated partial Hadamard matrices and related topics, Open Syst. Inf. Dyn. 25 (2018), 1–27.DOI
  3. [7]K. Beauchamp and R. Nicoara, Orthogonal maximal abelian *-subalgebras of the 6 × 6 matrices, Linear Algebra Appl. 428 (2008), 1833–1853.DOI
  4. [8]G. Björck, Functions of modulus 1 on Zₙ whose Fourier transforms have constant modulus, and cyclic n-roots, NATO Adv. Sci. Inst. Ser. C Math. Phys. Sci. 315 (1990), 131–140.DOI
  5. [9]G. Björck and R. Fröberg, A faster way to count the solutions of inhomogeneous systems of algebraic equations, with applications to cyclic n-roots, J. Symbolic Comput. 12 (1991), 329–336.DOI
  6. [10]A.T. Butson, Generalized Hadamard matrices, Proc. Amer. Math. Soc. 13 (1962), 894–898.DOI
  7. [12]P. Diţă, Some results on the parametrization of complex Hadamard matrices, J. Phys. A 37 (2004), 5355–5374.DOI
  8. [14]U. Haagerup, Orthogonal maximal abelian *-subalgebras of the n × n matrices and cyclic n-roots, in “Operator algebras and quantum field theory”, International Press (1997), 296–323.
  9. [20]T.Y. Lam and K.H. Leung, On vanishing sums of roots of unity, J. Algebra 224 (2000), 91–109.DOI
  10. [21]P. H. J. Lampio, P. R. J. Östergård, and F. Szöllősi, Orderly generation of Butson Hadamard matrices, Mathematics of Computation 89 (2020), 313–331.arxiv.org/abs/1707.02287
  11. [26]M. Petrescu, Existence of continuous families of complex Hadamard matrices of certain prime dimensions and related results, Ph.D. Thesis, UCLA (1997).
  12. [28]F.P. Ramsey, On a problem of formal logic, Proc. London Math. Soc. (2) 30 (1930), 264–286.
  13. [29]F. Szöllősi, Exotic complex Hadamard matrices and their equivalence, Cryptogr. Commun. 2 (2010), 187–198.DOI
  14. [31]W. Tadej and K. Życzkowski, A concise guide to complex Hadamard matrices, Open Syst. Inf. Dyn. 13 (2006), 133–177.DOI
  15. [32]W. Tadej and K. Życzkowski, Defect of a unitary matrix, Linear Algebra Appl. 429 (2008), 447–481.
  16. [33]T. Tao, Fuglede’s conjecture is false in 5 and higher dimensions, Math. Res. Lett. 11 (2004), 251–258.

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