This site will reopen in 2017. Until then, there is one article available:

Proof of the Irrationality of the Square Root of 2“, extended with exercises with solutions, 12 resp. 15 pages.

English versions: A4, A4-dup­lex, US-Let­ter, US-Let­ter-dup­lex.

German versions: A4, A4-dup­lex, US-Let­ter, US-Let­ter-dup­lex.