tailieunhanh - Báo cáo toán học: "On cycles in the coprime graph of integers1"

Tuyển tập các báo cáo nghiên cứu khoa học hay nhất của tạp chí toán học quốc tế đề tài: On cycles in the coprime graph of integers1. | On cycles in the coprime graph of integers1 Paul Erdos Mathematics Institute Hungarian Academy of Sciences Realtanoda u. 13-15 H-1053 Budapest Hungary Gabor N. Sarkozy Computer Science Department Worcester Polytechnic Institute Worcester MA 01609 gsarkozy@ Submitted June 20 1996 Accepted December 2 1996 Dedicated to Herbert S. Wilf on the occasion of his 65th birthday Abstract In this paper we study cycles in the coprime graph of integers. We denote by f n k the number of positive integers m n with a prime factor among the hrst k primes. We show that there exists a constant c such that if A c 1 2 . ng with I A f n 2 if 6 n then f n 2 2n then the coprime graph induced by A not only contains a triangle but also a cycle of length 2l 1 for every positive integer l cn. 1 Introduction Notations and definitions Let a b denote the greatest common divisor and a b the least common multiple of integers a b. Consider the coprime graph on the integers. This is the graph whose vertex set is the set of integers and two integers a b are connected by an edge if and only if a b 1. Let A c 1 2 . ng be a set of positive integers. The coprime graph of A denoted by G A is the induced coprime graph on A. A m u denotes the integers a 2 A ai u mod m . Á n denotes Euler s function n denotes the number of distinct prime factors of n and p n is the Moebius function. V G and E G denote the vertex set and edge set of a graph G. Kn is the complete graph on n vertices Cn is the cycle on n vertices and K m n denotes the complete bipartite graph between U and V where UI m VI n. H is a subgraph of G denoted by H c G if V H c V G and E H c E G . 1Mathematics Subject Classification 11B75 05C38 1 THE ELECTRONIC .JOURNAL OF COmBINATORICS 4 no. 2 1997 R8 2 The coprime graph of integers Recently the investigation of various graphs on the integers has received signihcant attention see . Graphs on the integers in 10 . The most popular graph seems to be the coprime graph altough there .

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