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On the Asymptotic Existence of Partial Complex Hadamard Matrices And Discrete Applied Mathematics 102 (2000) 37–45 View metadata, citation and similar papers at core.ac.uk brought to you by CORE On the asymptotic existence of partial complex Hadamard provided by Elsevier - Publisher Connector matrices and related combinatorial objects Warwick de Launey Center for Communications Research, 4320 Westerra Court, San Diego, California, CA 92121, USA Received 1 September 1998; accepted 30 April 1999 Abstract A proof of an asymptotic form of the original Goldbach conjecture for odd integers was published in 1937. In 1990, a theorem reÿning that result was published. In this paper, we describe some implications of that theorem in combinatorial design theory. In particular, we show that the existence of Paley’s conference matrices implies that for any suciently large integer k there is (at least) about one third of a complex Hadamard matrix of order 2k. This implies that, for any ¿0, the well known bounds for (a) the number of codewords in moderately high distance binary block codes, (b) the number of constraints of two-level orthogonal arrays of strengths 2 and 3 and (c) the number of mutually orthogonal F-squares with certain parameters are asymptotically correct to within a factor of 1 (1 − ) for cases (a) and (b) and to within a 3 factor of 1 (1 − ) for case (c). The methods yield constructive algorithms which are polynomial 9 in the size of the object constructed. An ecient heuristic is described for constructing (for any speciÿed order) about one half of a Hadamard matrix. ? 2000 Elsevier Science B.V. All rights reserved. 1. A result in analytic number theory In 1742 Goldbach wrote to Euler conjecturing that every integer greater than 5 can be written as the sum of exactly 3 primes. Euler wrote back saying that Goldbach’s conjecture is true if and only if every even integer greater than 3 is the sum of exactly 2 primes. 1 In 1937, Vinogradov [5] proved that, for some ÿxed integer N, every odd integer n>N can be written as the sum of 3 primes. According to Ribenboim [4], we may take N =3315 . In 1990, Pan Chengdong and Pan Chengbiao [3] proved the following result. E-mail address: [email protected] (W. de Launey) 1 Nowadays, presumably because of Vinogradov’s theorem, Euler’s reformulation of Goldbach’s original conjecture is generally called the Goldbach Conjecture. 0166-218X/00/$ - see front matter ? 2000 Elsevier Science B.V. All rights reserved. PII: S0166-218X(99)00229-2 38 W. de Launey / Discrete Applied Mathematics 102 (2000) 37{45 Theorem 1.1. There is an absolute positive integer c such that for U(n)=n2=3(log n)c; the number T(n) of triples of primes p1;p2;p3 which obey the Diophantine constraints n = p1 + p2 + p3; (1.1) n=3 − U(n) <pj <n=3+U(n);j=1;2;3; is given by the asymptotic identity T(n)=3S(n)U(n)2(log n)−3 + O((log n)−4); where Y 1 Y 1 S (n)= 1− 1+ : (p−1)2 (p−1)3 p|n p|n Note that when n is even, S(n) = 0, whereas when n is odd, it is well known that 1 S(n) > 2 . So when n is even all we can deduce from the formula for T is that T grows fairly slowly, whereas, when n is odd, we can be sure that T eventually stays positive. Thus Theorem 1.1 implies that Goldbach’s conjecture is asymptotically true for odd n, but says little about the conjecture for even n. Notwithstanding its importance in number theory, Theorem 1.1 has implications in combinatorial design theory because it proves that for all suciently large odd integers the three primes summing to n can be chosen to be close to n=3. The proof of Theorem 1.1 is a lengthy calculation using estimates of various functions arising in analytic number theory. The somewhat weaker result Proposition 1.2. For any ¿0; there is an integer N such that for all odd integers n>N; there exist three primes p1;p2;p3 satisfying n = p1 + p2 + p3; 1 1 (1.2) n − <p <n + ;j=1;2;3: 3 j 3 This result can be obtained via a straightforward adaptation of an argument devel- oped in [1, Sections 24–26] to prove Vinogradov’s result. Although using this result would sacriÿce some precision, it is really all that is needed to obtain results with the main features of those given in this paper, and there is the advantage that the ideas needed to understand the proof of Proposition 1.2 can be found in one well written book. Davenport’s argument depends on the prime number theorem for arithmetic pro- gressions. Let A be any positive number. The adaptation needed to prove Proposition 1.2 begins by aiming to estimate the quantity X S1( )= (k)e(k ) 1 −A |k−3N|6N(log N) W. de Launey / Discrete Applied Mathematics 102 (2000) 37{45 39 (where is Von Mangoldt’s function) in place of X S( )= (k)e(k ): k6N The goal of the argument is to show that the integral Z 1 X 3 r1(N)= S1( ) e(−N )d = (k1)(k2)(k3) 0 k1+k2+k3=N −A |ki−1=3N|6N(log N) is positive for suciently large odd N. The bulk of Davenport’s e ort is directed at showing that most of the range of his integral does not matter, and that, in the part of the range that does matter, the dominant term is essentially proportional to the number of ways of writing N as the sum of three positive integers k1;k2;k3 (see p. 148 of the second edition). This number is clearly (N − 1)(N − 2)=2. If one asks the analogous −A −A 2 question for ki such that |ki −N=3|6N(log N) , then one gets 3(N=(log N) ) which is still large enough to be the dominant part of the integral r1(N). 2. Asymptotic existence of partial complex Hadamard matrices Deÿnition 2.1. An m×n partial complex Hadamard matrix is an m×n (1; i)-matrix A satisfying ∗ AA = nIm: (2.1) Lemma 2.2. If there is an m × n partial complex Hadamard matrix for n>1; then n is even. Proof. Pick any two distinct rows. The inner product of these two rows is of the form a1 + a2i − a3 − a4i where n = a1 + a2 + a3 + a4. Since the inner product is 0, we have a1 = a3 and a2 = a4. Hence, n =2(a1 +a3). Note that if m = n, then (1=n)A∗ is the inverse of A. It follows that there are no vectors in the null space of A except the zero vector. Therefore, m cannot exceed n. When m = n, the matrix A is called a complex Hadamard matrix of order n.Ithas been conjectured that there is a complex Hadamard matrix of order n for all even n. The following result shows that for suciently large n, there exists at least about one third of a complex Hadamard matrix of order n. Theorem 2.3. There is an integer N such that for all t>N; there is a d((2=3)t − U(2t − 3))e×2t partial complex Hadamard matrix. Proof. By Theorem 1.1, we may write 2t =1+p1 +1+p2 +1+p3; 40 W. de Launey / Discrete Applied Mathematics 102 (2000) 37{45 where p3¿p2¿p1¿(2=3)t − U(2t − 3) are odd primes. For j =1;2;3 let Cj be the Paley conference matrix of order 1 + pj, and set ( iI + C if p ≡ 1 (mod 4); 1+pj j j Dj = + C if p ≡ 3 (mod 4): I1+pj j j For j =1;2;3 let Ej be the (1 + p1) × (1 + pj) matrix obtained by taking the ÿrst 1+p1 rows of Dj. Then for j =1;2;3 we have E∗ =(1+p)I Ej j j 1+p1 and the matrix B =[E1 |E2 |E3] (2.2) satisÿes ∗ =2tI : BB 1+p1 The theorem now follows. 3. Asymptotic existence of partial Hadamard matrices Deÿnition 3.1. An m × n partial Hadamard matrix is an m × n (1; −1)-matrix H satisfying > HH = nIm: (3.1) Note that we may negate any row or column of H without disrupting Eq. (3.1). So we may change to 1 every entry in the ÿrst row or column of H by negating appropriate rows and columns. Once this is done we say H is normalized. The following result was published by Hadamard in 1893. Lemma 3.2. If H =(hij) is a normalized m×n partial Hadamard matrix with m>2; then for all integers i1 and i2 such that 1 <i1 <i26m the sequence of ordered pairs ;h );(h ;h (hi11 i21 i12 i22);:::;(hi1n;hi2n) contains each of the ordered pairs (1; 1) exactly n=4 times. In particular; if m>2; then 4 divides n. Now in the same way as for partial complex Hadamard matrices, we may prove that m cannot exceed n.Ifm=n, then H is a Hadamard matrix of order n. In 1893, Hadamard conjectured that such matrices exist for all orders divisible by four. A stan- dard method for converting from complex Hadamard matrices to Hadamard matrices allows us to prove that for suciently large n we can construct at least about one third of Hadamard matrix of order n.
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