ELECTRONICS LETTERS, vol. 29, no. 14, page. 1281 - 1282, 1993-07-08
ELECTRONICS LETTERS, vol. 30, no. 19, page. 1592 - 1593, 1994-09-15
INFORMATION PROCESSING LETTERS, vol. 53, no. 2, page. 91 - 96, 1995-01-27
ELECTRONICS LETTERS, vol. 31, no. 10, page. 785 - 786, 1995-05-11