AMERICAN MATHEMATICAL MONTHLY, vol. 112, no. 7, page. 585 - 601, 2005-08
JOURNAL OF MATHEMATICAL ANALYSIS AND APPLICATIONS, vol. 281, no. 2, page. 417 - 424, 2003-05-15
ILLINOIS JOURNAL OF MATHEMATICS, vol. 45, no. 4, page. 1273 - 1299, 2001-01