tailieunhanh - Báo cáo toán học: "On the failing cases of the Johnson bound for error-correcting codes"

Tuyển tập các báo cáo nghiên cứu khoa học về toán học trên tạp chí toán học quốc tế đề tài: On the failing cases of the Johnson bound for error-correcting codes. | On the failing cases of the Johnson bound for error-correcting codes Wolfgang Haas Albert-Ludwigs-Universitat Mathematisches Institut Eckerstr. 1 79104 Freiburg Germany wolfgang_haas@ Submitted Mar 4 2008 Accepted Apr 4 2008 Published Apr 18 2008 Mathematics Subject Classification 94B25 94B65 Abstract A central problem in coding theory is to determine Aq n 2e 1 the maximal cardinality of a q-ary code of length n correcting up to e errors. When e is fixed and n is large the best upper bound for A n 2e 1 the binary case is the well-known Johnson bound from 1962. This however simply reduces to the sphere-packing bound if a Steiner system S e 1 2e 1 n exists. Despite the fact that no such system is known whenever e 5 they possibly exist for a set of values for n with positive density. Therefore in these cases no non-trivial numerical upper bounds for A n 2e 1 are known. In this paper the author presents a technique for upper-bounding Aq n 2e 1 which closes this gap in coding theory. The author extends his earlier work on the system of linear inequalities satisfied by the number of elements of certain codes lying in k-dimensional subspaces of the Hamming Space. The method suffices to give the first proof that the difference between the sphere-packing bound and Aq n 2e 1 approaches infinity with increasing n whenever q and e 2 are fixed. A similar result holds for Kq n R the minimal cardinality of a q-ary code of length n and covering radius R. Moreover the author presents a new bound for A n 3 giving for instance A 19 3 26168. 1 Introduction In the whole paper let q denote an integer greater than one and Q a set with IQ I q. The Hamming distance d A p between A x1 . xn 2 Qn and p y-1 . yn 2 Qn is defined by d A p i 2 1 . ng xi yig . THE ELECTRONIC JOURNAL OF COMBINATORICS 15 2008 R55 1 Let Bq A e denote the Hamming sphere with radius e centered on A 2 Qn Bq A e p 2 Qn d p A eg. We set Vq n e Bq A e X q- ứ 0 i e A and Vq n e p 2 Qn d p A eg for any A 2 Qn. Assume