Puppel, Stanisław. „An Assessment of Natural Language Robustness and How It Relates to Natural Language Sustainability: A Fuzzy Ecological Approach to the Lives of Natural Languages”. Studia Rossica Posnaniensia, no. 38 (grudzień 31, 2013): 223–230. Dostęp grudzień 10, 2025. https://pressto.amu.edu.pl/index.php/strp/article/view/354.