from Cordon Art B.V.
http://national.gallery.ca/slidekits/slidekit_escher
http://www.etropolis.com/escher/index.html
http://www.nga.gov/collection/gallery/ggescher/ggescher-main1.html
http://www.mathacademy.com/platonic_realms/minitext/escher.html
http://www.djmurphy.demon.co.uk/escher.htm
http://users.erols.com/ziring/escher_bio.htm
http://www.geocities.com/SoHo/Museum/3828/home.html
http://www.worldofescher.com
http://artforkids.about.com/kids/artforkids/library/weekly/aa071697.htm