BOUNDS FOR THE SIZE OF SETS WITH THE PROPERTY D(n) Andrej Dujella University of Zagreb, Croatia

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1 GLASNIK MATMATIČKI Vol. 39(59(2004, BOUNDS FOR TH SIZ OF STS WITH TH PROPRTY D(n Andrej Dujella University of Zagreb, Croatia Abstract. Let n be a nonzero integer and a 1 < a 2 < < a m ositive integers such that a i a j +n is a erfect square for all 1 i < j m. It is known that m 5 for n = 1. In this aer we rove that m 31 for n 400 and m < log n for n > Introduction Let n be a nonzero integer. A set of m ositive integers {a 1, a 2,..., a m } is called a D(n-m-tule (or a Diohantine m-tule with the roerty D(n if a i a j + n is a erfect square for all 1 i < j m. Diohantus himself found the D(256-quadrule {1, 33, 68, 105}, while the first D(1-quadrule, the set {1, 3, 8, 120}, was found by Fermat (see [4, 5]. In 1969, Baker and Davenort [1] roved that this Fermat s set cannot be extended to a D(1-quintule, and in 1998, Dujella and Pethő [10] roved that even the Diohantine air {1, 3} cannot be extended to a D(1-quintule. A famous conjecture is that there does not exist a D(1-quintule. We roved recently that there does not exist a D(1-sextule and that there are only finitely many, effectively comutable, D(1-quintules (see [7, 9]. The question is what can be said about the size of sets with the roerty D(n for n 1. Let us mention that Gibbs [12] found several examles of Diohantine sextules, e.g. {99, 315, 9920, 32768, 44460, } is a D( sextule. Define M n = su{ S : S has the roerty D(n} Mathematics Subject Classification. 11D45, 11D09, 11N36. Key words and hrases. Diohantine m-tules, roerty D(n, large sieve. 199

2 200 A. DUJLLA Considering congruences modulo 4, it is easy to rove that M n = 3 if n 2 (mod 4 (see [3, 13, 15]. On the other hand, if n 2(mod 4 and n { 4, 3, 1, 3, 5, 8, 12, 20}, then M n 4 (see [6]. In [8], we roved that M n 32 for n 400 and M n < log n (log log n 2 for n > 400. The urose of the resent aer is to imrove this bound for M n, secially in the case n > 400. We will remove the factor (log log n 2, and also the constants will be considerably smaller. The above mentioned bounds for M n were obtained in [8] by considering searately three tyes (large, small and very small of elements in a D(n-mtule. More recisely, let A n B n C n = su{ S [ n 3, + : S has the roerty D(n}, = su{ S n 2, n 3 : S has the roerty D(n}, = su{ S [1, n 2 ] : S has the roerty D(n}. In [8], it was roved that A n 21 and B n < 0.65 log n for all nonzero integers n, while C n < log n (log log n log log n for n > 400 and C n 5 for n 400. The combination of these estimates gave the bound for M n. In the estimate for A n, a theorem of Bennett [2] on simultaneous aroximations of algebraic numbers was used in combination with a ga rincile, while a variant of the ga rincile gave the estimate for B n. The bound for C n (number of very small elements was obtained using the Gallagher s large sieve method [11] and an estimate for sums of characters. In the resent aer, we will significantly imrove the bound for C n using a result of Vinogradov on double sums of Legendre s symbols. Let us mention that Vinogradov s result, in a slightly weaker form, was used recently, in similar context, by Gyarmati [14] and Sárközy & Stewart [17]. We will rove the following estimates for C n. Proosition 1.1. If n > 400, then C n < log n. If n , then C n < 8.37 log n. More detailed analysis of the ga rincile used in [8] will lead us to the slightly imroved bounds for B n. Proosition 1.2. For all nonzero integers n it holds B n < log n If n > 400, then B n < log n By combining Proositions 1.1 and 1.2 with the above mentioned estimate for A n, we obtain immediately the following estimates for M n. Theorem 1.3. If n 400, then M n 31. If n > 400, then M n < log n. If n , then M n < log n.

3 TH SIZ OF STS WITH TH PROPRTY D(n Three lemmas Lemma 2.1 (Vinogradov. Let be an odd rime and gcd(n, = 1. If A, B {0, 1,..., 1} and T = ( xy + n, x A y B then T < A B. Proof. See [18, Problem V.8.c]. Lemma 2.2 (Gallagher. If all but g( residue classes mod are removed for each rime in a finite set S, then the number of integers which remain in any interval of length N is at most ( /( log (2.1 log log N g( log N S rovided the denominator is ositive. Proof. See [11]. Lemma 2.3. If {a, b, c} is a Diohantine trile with the roerty D(n and ab + n = r 2, ac + n = s 2, bc + n = t 2, then there exist integers e, x, y, z such that ae + n 2 = x 2, be + n 2 = y 2, ce + n 2 = z 2 and Proof. See [8, Lemma 3]. S c = a + b + e n + 2 (abe + rxy. n2 3. Proof of Proosition 1.1 Let N n 2 be a ositive integer. Since n > 400, we have N > Let D = {a 1, a 2,..., a m } {1, 2,..., N} be a Diohantine m-tule with the roerty D(n. We would like to find an uer bound for m in term of N. We will use the Gallagher s sieve (Lemma 2.2. Let S = { : is rime, gcd(n, = 1 and Q}, where Q is sufficiently large. For a rime S, let C denotes the set of integers b such that b {0, 1, 2, (..., 1} and there is at least one a D such that b a(mod. Then xy+n {0, 1} for all distinct x, y C. Here ( ( denotes the Legendre symbol. If 0 C, then = 1. For a given. n

4 202 A. DUJLLA x C \ {0}, we have = 1. Therefore, ( xy+n T = ( xy0+n = 0 for at most one y 0 C. If y x, y 0, then x,y C ( xy + n = ( ( xy + n x C y C x C( C 3 C ( C 3. On the other hand, Lemma 2.1 imlies T < C. Thus, C < + 3 and we may aly Lemma 2.2 with g( = min{ + 3, }. Let us denote the numerator and denominator from (2.1 by and F, resectively. By [16, Theorem 9], we have = S log log N < θ(q < Q. The function f(x = log x min{ x+3,x} is strictly decreasing for x > 25. Also, if Q 118, then f( f(q for all Q. For S it holds gcd(n, = 1. This condition comes from the assumtions of Lemma 2.1. However, we will show later that n can be divisible only by a small roortion of the rimes Q. Assume that n is divisible by at most 5% of rimes Q. Then, for Q 118, we have (3.1 F f( log N S log Q 19 Q log Q Q + 3 S log N π(q log N > 0.95 Q Q + 3 log N. Since F has to be ositive in the alications of Lemma 2.2, we will choose Q of the form Q = c 1 log 2 N. We have to check whether our assumtion on the roortion of rimes which divide n is correct. Suose that n is divisible by at least 5% of the rimes Q. Then n 1 2 π(q/20, where i denotes the i-th rime. By [16, 3.5 and 3.12], we have π(q/20 > R, where R = 1 Q ( 1 20 log Q log Q. 20 log Q

5 TH SIZ OF STS WITH TH PROPRTY D(n 203 Assume that c 1 6. Then Q > 860 and R > From [16, 3.16], it follows that (3.2 log n > ( log > R log R R Q 1 Furthermore, 20 log Q > Q0.273 and R > Q. Hence, (3.2 imlies log R > and therefore log N 2 log n > Q log 2 N, contradicting the assumtion that N > Therefore, we have that n is divisible by at most 5% of the rimes Q, and hence we have justified the estimate (3.1. Under the assumtion that c 1 6, the inequality (3.1 imlies F > Q log N = (0.861 c 1 1 log N and F < c log N. c 1 1 For c 1 = 6 we obtain (3.3 < log N. F Assume now that N and c 1 4. Then Q > and we can rove in the same manner as above that n is divisible by at most 1% of the rimes Q. This fact imlies F < 1.017c log N. c 1 1 For c 1 = 4.11 we obtain (3.4 < log N. F Setting N = n 2 in (3.3 and (3.4, we obtain the statements of Proosition Proof of Proosition 1.2 We may assume that n > 1. Let {a, b, c, d} be a D(n-quadrule such that n 2 < a < b < c < d. We aly Lemma 2.3 on the trile {b, c, d}. Since b > n 2 and be + n 2 0, we have that e 0. If e = 0, then d = b + c + 2 bc + n < 2c + 2 c(c 1 + n < 4c, contradicting the fact that d > 4.89 c (see [8, Lemma 5]. Hence e 1 and (4.1 d > b + c + 2bc n 2 + 2t bc n 2.

6 204 A. DUJLLA Lemma 2.3 also imlies (4.2 c a + b + 2r. From r 2 ab 4 ab and ab 30 it follows that r > 0.96 a, and (4.2 imlies c > 3.92 a. Similarly, bc 42 imlies t > bc and, by (4.1, d > b + c bc n > c + b. 2 Assume now that {a 1, a 2,..., a m } is a D(n-m-tule and n 2 < a 1 < a 2 < < a m < n 3. We have a 3 > 3.92 a 1, a i > a i 1 + a i 2, for i = 4, 5,..., m. Therefore, a m > α m a 1, where the sequence (α k is defined by (4.3 α k = 4.938α k 1 + α k 2, α 2 = 1, α 3 = Solving the recurrence (4.3, we obtain α k βγ k 3, with β , γ More recisely, α k βγ k 3 < 1 βγ k 3. From n 3 1 a m > α m a 1 α m (n 2 + 1, it follows α m n 1 n βγ m 3 < n. Hence, (4.4 m < 1 log β log n + 3 log γ log γ. For the above values of β and γ we obtain m < log n Assume now that n > 400. Then bc > ab > 400 4, which imlies c > a and d > c + b. Therefore, in this case the relation (4.4 holds with β , γ , and we obtain m < log n and Remark 4.1. The constants in Theorem 1.3 can be imroved, for large n, by using formula (2.26 from [16] in the estimate for the sum S f(. In that way, it can be roved that for every ε > 0, F > (2 ε Q log N holds for sufficiently large Q. Also, in the roof of Proosition 1.2, for sufficiently ( large n we have c > 1 (4 εa and d > (5 εc + b, which leads to B n < log( ε log n. These results imly that for every ε > 0 there exists n(ε such that for n > n(ε it holds M n < ( log( ε log n.

7 TH SIZ OF STS WITH TH PROPRTY D(n 205 Acknowledgements. The author is grateful to the referees for valuable comments, and in articular for ointing out a ga in the roof of Proosition 1.1 in the first version of the manuscrit. References [1] A. Baker and H. Davenort, The equations 3x 2 2 = y 2 and 8x 2 7 = z 2, Quart. J. Math. Oxford Ser. (2 20 (1969, [2] M. A. Bennett, On the number of solutions of simultaneous Pell equations, J. Reine Angew. Math. 498 (1998, [3]. Brown, Sets in which xy + k is always a square, Math. Com. 45 (1985, [4] L.. Dickson, History of the Theory of Numbers, Vol. 2, Chelsea, New York, 1966, [5] Diohantus of Alexandria, Arithmetics and the Book of Polygonal Numbers, (I. G. Bashmakova, d., Nauka, Moscow, 1974 (in Russian, , 232. [6] A. Dujella, Generalization of a roblem of Diohantus, Acta Arith. 65 (1993, [7] A. Dujella, An absolute bound for the size of Diohantine m-tules, J. Number Theory 89 (2001, [8] A. Dujella, On the size of Diohantine m-tules, Math. Proc. Cambridge Philos. Soc. 132 (2002, [9] A. Dujella, There are only finitely many Diohantine quintules, J. Reine Angew. Math. 566 (2004, [10] A. Dujella and A. Pethő, A generalization of a theorem of Baker and Davenort, Quart. J. Math. Oxford Ser. (2 49 (1998, [11] P. X. Gallagher, A larger sieve, Acta Arith. 18 (1971, [12] P. Gibbs, Some rational Diohantine sextules, rerint, math.nt/ [13] H. Guta and K. Singh, On k-triad sequences, Internat. J. Math. Math. Sci. 5 (1985, [14] K. Gyarmati, On a roblem of Diohantus, Acta Arith. 97 (2001, [15] S. P. Mohanty and A. M. S. Ramasamy, On P r,k sequences, Fibonacci Quart. 23 (1985, [16] J. B. Rosser and L. Schoenfeld, Aroximate formulas for some functions of rime numbers, Illinois J. Math. 6 (1962, [17] A. Sárközy and C. L. Stewart, On rime factors of integers of the form ab + 1, Publ. Math. Debrecen 56 (2000, [18] I. M. Vinogradov, lements of Number Theory, Nauka, Moscow, 1972 (in Russian. A. Dujella Deartment of Mathematics University of Zagreb Bijenička cesta 30, Zagreb Croatia -mail: duje@math.hr Received: Revised: