http://msp.org/apde/2012/5-2/apde-v5-n2-p07-s.pdf
http://www.ems-ph.org/journals/show_pdf.php?issn=1661-7207&vol=5&iss=2&rank=6
http://www.ems-ph.org/journals/show_pdf.php?issn=0010-2571&vol=88&iss=1&rank=3
http://prd.aps.org/pdf/PRD/v13/i12/p3491_1