Hi, PEN FRIENDS!
Here is the pdf file of PEN extended (version 0.9)! Lots of interesting problems from recent mathematics olympiads are available, YAY! Good problems from this book will be taken, and included to the original PEN Problems Book. If you find any typos or errors, then please email us at firstname.lastname@example.org
PEN Problems Book has 649 problems and PEN extended has 337 problems!
– PEN TEAM
One more thing. We have a Project PEN Group on facebook. Anyone who loves number theory is welcome!