As a publishing house, we have become a member of the American Crossref association. Because of it we can already assign DOI numbers to our articles. All future articles (from the issue 5/2022) will already have DOI number automatically. In the case of earlier articles – if the authors are interested – please contact us by e-mail to the address firstname.lastname@example.org. The cost of assigning a DOI number (only articles from 2006-2022) is PLN 30.