Methods for the formation of digital collections of a digital mathematical library are presented. With the help of tools for analyzing the structure of documents and their style features, the main set of document metadata has been formed. For each document, this set includes the title of the article, a list of authors, and a list of cited bibliography. To supplement the metadata, methods of extracting knowledge from Wikidata were used. With the help of the developed system of SPARQL-queries, the search and refinement of data on documents of the collections was carried out. In particular, information about the authors of the articles has been added (full spelling of surnames, first names, patronymics in various languages, place of work at the time of writing the article, etc.). In addition, methods are proposed for refining and supplementing bibliographic references given in the articles. When forming metadata of retro collections, a search was made in Wikidata for information about the years of life of authors of articles, URLs of web pages with information about articles and their authors. The results of the several digital collections formation, which are included in the digital library Lobachevskii-DML, are presented.