Luck, L., List, H., Middell, G., & Hybler, Z. (1990). English step by step: English for beginners and an elementary refresher course, English for intermediate learners, English for advanced learners and scientiests (8., unveränd. Aufl.). Verlag Enzyklopädie.
توثيق أسلوب شيكاغو (الطبعة السابعة عشر)Luck, Lydia, Horst List, Gudrun Middell, و Zdenek Hybler. English Step by Step: English for Beginners and an Elementary Refresher Course, English for Intermediate Learners, English for Advanced Learners and Scientiests. 8., unveränd. Aufl. Leipzig: Verlag Enzyklopädie, 1990.
توثيق جمعية اللغة المعاصرة MLA (الطبعة الثامنة)Luck, Lydia, et al. English Step by Step: English for Beginners and an Elementary Refresher Course, English for Intermediate Learners, English for Advanced Learners and Scientiests. 8., unveränd. Aufl. Verlag Enzyklopädie, 1990.