- http://www.hueber.de/shared/uebungen/schritte-international/fset.php?Volume=1&Lection=6&Exercise=2&SubExercise=1&ExerciseUrl=schr1intL6ex2.htm
- http://62.48.71.204/shared/uebungen/pingpong/lerner/uebungen/fset.php?Volume=1&Lection=11&Exercise=5&SubExercise=1