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The Skew - Weighing Matrix Conjecture

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The Skew - Weighing Matrix Conjecture University of Wollongong Research Online Faculty of Engineering and Information Faculty of Informatics - Papers (Archive) Sciences 1982 The skew - weighing matrix conjecture Jennifer Seberry University of Wollongong, [email protected] Follow this and additional works at: https://ro.uow.edu.au/infopapers Part of the Physical Sciences and Mathematics Commons Recommended Citation Seberry, Jennifer: The skew - weighing matrix conjecture 1982. https://ro.uow.edu.au/infopapers/1008 Research Online is the open access institutional repository for the University of Wollongong. For further information contact the UOW Library: [email protected] The skew - weighing matrix conjecture Abstract We review the history of the skew-weighing matrix conjecture and show that there exist skew-symmetric weighing matrices W (21.2t, k) for all k=0,l,.....,21.2t - I, t ≥ 4 a positive integer. Hence there exist orthogonal designs of type l(l,k) for all k=O,l,..., 21.2t - 1, t ≥ 4 a positive integer, in order 21.2t. Disciplines Physical Sciences and Mathematics Publication Details Seberry, J, The skew - weighing matrix conjecture, University of Indore Research J. Science, 7, 1982, 1-7. This journal article is available at Research Online: https://ro.uow.edu.au/infopapers/1008 , , The Skew - Weighing Matrix Conjectnre Jennifer Sebetry* SUMMARY, We review the history of the skew.weighing matrlx conjecture and show that there exist skew-symmetric weighing matrices W (21.2t, k) for all k=O,l, ......• 21.2' - I, t ~ 4 a positive integer. Hence there exist orthogonal designs of type l(l,k) for all k=O,l • .. " 2L2~ - 1, t ~ 4 a positive integer, in order 21.2'. 1 . Introduction: An orthogonal design X of order n and type (Sl. S2, .. " St) on the commuting variables 0, Xl> .•• , x~ is a square matrix with entries from {D, ± Xl. •••• ± x,} with the properties that ( i) the element ± XI occurs s\ times in each row and column, (ii) the inner product of distinct rows, is zero. Hence X satisfies XXT = ')' SIX~i In ~ i= 1 *This paper was written while visiting the Department of statistics, University of Indore, Indore. where Iu is the identity matrix. A weighing matrix W= W (n,k) is a square matrix with entries 0, ± 1 having k non­ zero entries per row and column and inner product of distinct rows zero. Hence W satisfies. WWT = kIn. and W is equivalent to an orthogonal design of order n and type (k). The number k is called the weight of W. Weighing matrices have long been studied because of their ~se in weighing experiments as first stUdied by HoteHing (1944) and latter by Raghavarao (1960) and others. In Wallis (1972) it was conjectured that if n;;::D (mod 4) then weighing mat:-ices W (n, k) exist for every k= 1,2 .... n. Call this Conjecture A. This was proved true for orders n=2t, t a positive integer in Gerammita, Pullman. Wallis (1974). Later the conje:;ture was made stronger by Seberry until it appeared in the following form: Conje=ture B: There e:~ist skew-symmetric weighing matrices ~v (n,k) for ever~ k=l.2 ... , n-\ when lIaO (mod 8). Equivale:J.tly. every orthogonal design (1..;;), k=l,= . ... , n-t exists when n;;::D (mod 8). This conjecture was established for n=2t.3, 2t.5. 2'.9 by Geramita and Wallis (1974. 1975), for n=2'.7 by Ead~s and Wallis (1976) and for n=2t'l""1.lS by Seberry (1980). f;<: 3, an integer. In this paper we will estahlish the conjecture for n=2t"t'1 ..21. ... This polper was written while visiting the Department of Statistics, University of Indore.. Indore. 2 INDORE UNIVERSITY 2. PrelimiDary Results And Notaion We make extensive use of the book Geramita and Se'berry (1979). We quote the following theorems, giving their reference from the af~rementioned book, that we use : Lemma 1 : (Lemma 4.11). If there exists an orthogonal design in order rt of type (sJ,.I'z; .. " sJ then there exists an orthogonal design in oruer 2n of type (SI,shes2 • ...• esu). e= 1 or 2. Lemma 2: (umma 4.4), If A is an orthogonal design of order rt and type (Uh •• " us) on the variables .1'10 •• " X$. then there is an orthogonal design of order nand types Cut. ...• Uj+Uj, •• " us) and (UI> .• " Ut, ••• , UJ_h Uj-f 1•••• , us) on the .1'-1 variables Xl> .... Xj' ""XI' Example: X y = -y x w -z -, -w )' [ -w = -y x ] is an orthogonal design of type: (1, I. 1. 1) in order 4. U~.i:lg Lemma 2 we see x y 0 w -yXY'.] x ; -; and [ -y x w 0 -z -z x y 0 -w x y [ -z z -y x -w 0 -y x ] are orthogonal designs of types (1,1,2) and (1,1,1) respectively. Lemma 1 tells US there are otherogonal designs of types (1,1,2,2,2,), (1,1,2,4), (2,2,2,2,) and (2,2,2,) in order 8. Lemma 3 (Corollary 5.2): If all orthogonal designs (1, k), k=l, ...• n-l, exi::.t in order n, then all orthogonal designs {l,j),j= 1, .. " 2n-l, exist in order 2n. Example: Using the orthogonal design of type (1, 1, 1, 1) in order 4 with Lemma 2 we see that orthogonal designs of types (I, 1), (1,2) and (1, 3) exist in order 4. Lemma 3 is now used to see that orthogonal designs of types (1, k), k=J, 1, ...• 7 exist in order 8. By repeated use of Lemma 3 we see thut orthogonal designs of types(l,),j= 1, 2 •.. " 2t-l, 1 :l positive integer, exist in every order 2t , Theorem 4 (Theorems 2.19 and 2.20) : suppose n;;::::O (mod 4). Then the existence of a W (n, n-l) implies the existence of a skew·symmetdc W (n, n -\). The existence of a skew-symmetric W (n. k) is equivalent to the existence of an orthogonal design of types (l,k) in order n. Theorem 5 (Proposition 3.54 and Theorem 2.20) : An orthogonal design of type (1. k) can only ell.ist in order n:=:4 (mod 8) if k is the SUm of three squares. An orthogonal design of type (1, n~2) can only exist in order- n:=:4 (mod 8) if n-2 is the sum of two squares. Theorem 6: Orthogonal designs of types (t. k) exist for k=l • ...• n-1 in orders n=2t.1~+3. 3. 2''"1"3. 5. 2t+3. 7, 2'+~. 9, 2t+t. 15 t?O an integer. Theorem 7: (Theorem 4.49) If there exist four circulant matrices At. A2, As. A. of order n satisfying. RESEARCH JOURNA.L 3 4 L. A!AjT = fl i= I s where lis the quadratic form L. ljx2j • then there is an orthogonal design of order j=l Theorem 8: (Theorems 4.124 and 4.41). Let q be a prime power thea there is a circulant W (q2+q+ 1, qll). Let p= 1 (mod 4) then there are two circulant symmetric matric(!S R, S of order! (P+1) satisfying RRT + SST = PI, and a symmetric matrix P, with zero diagonal satisfying PP'1'=pl p+l that is a symmetric W(p+l, pl· Corollary 9: There exists a circulant W(21, 16) and W (7, 4). There exist two circulant symmetric matrices R. S of order n=41, 21 and 7 satisfying RRT + SST = (2n-I) In. There exist symmetric W (6,5) = X and W (l4,13) = Y with zere diagonal. We use the follo~'iDg notatioD : I is the identity matrix with order taken from the context, r is the matrix of ones with order taken from the context; A is the back..circulant matrix with first row b a a; B is the back.. circulant matrix with first row b a a :i a a a ; W7 is the back..circulant matrix with first row 11 1 0 I 0 0; Wz! is the back-circulant matrix with first row the same as the circulant W (21,16); Ru, Su are circulant symmetric matrices of order n=7, 21, 41 satisfying Sn SuT+Rn T Rn = (211-1) 1D; X is the symmetric W(6, 5); • y is the symmetric W(l4, 13); U, Y, Z are the circulant symmetric matrices with first rows 01 I - -1, 1,0 I - I 1 - I, 0 - 1 1 1 1 - respectively, UZ + y2 + Z2 = 19 I-I. 3. Resul1'i In Orders Di"risible By 21 : We recal! from Theorem 5 that orth')gonal d~igns of type (1, k) can only exist in order 84 if k is the sum of three squares. We see 84-2 = 92 + 12 so the other condition is satisfied.. Hence we have (1, k) can not exist for k=4a (8b+ 7), i.e. k E{7, 1'5,23. 28, 31,39,47, 55, 60, 63, 71. 79}. Theorem 10 : Orthogonal designs of types (I. k) exist in order 84 for k E {x/x = a2 +b2 +c2, x? 42, 61, 62, 67, 70, 73-78. 82}. Proof: Theorem 6 and Lemma 8.36 of Geramita and Seberry (1979) say that all orthogonal designs of type (1. k) exist in order 40 and 44 for k ~ 38 the sum of three integer squares. We have by direct sum, A @ B, of the OD of type (I, k) in order 40, A and the 0') of type (1, k) in order 44, B, the result in order 84.
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