ROCKY MOUNTAIN JOURNAL OF MATHEMATICS, vol. 39, no. 2, page. 455 - 462, 2009-04
PROCEEDINGS OF THE AMERICAN MATHEMATICAL SOCIETY, vol. 135, no. 8, page. 2309 - 2315, 2007-01
JOURNAL OF NUMBER THEORY, vol. 122, no. 1, page. 25 - 36, 2007-01