pickup - 111009

■Double Ut (by 山本長徳)
http://www.asahi-net.or.jp/~RH5K-ISN/Puzzle/553Ring/DoubleUt/

(via ちょいとパズルでも)

■Mathematical Puzzling [paperback]
Anthony Gardiner (Dover Publications)
http://www.amazon.co.jp/dp/0486409201/