A method of forming a mandatory set of metadata for retro collections of a digital mathematical library is presented. The open resources of the Semantic Network were used as a source for completing metadata. With the help of the software tools of the metadata factory of the digital mathematical library Lobachevskii-DML, the main processes of text analysis of documents of digital retro collections are performed, in particular, the selection of named entities. Further, through the system of queries in the semantic network, the search and selection of information objects is carried out. After performing automatic filtering and normalization, the obtained information is included in the metadata set. As one of the results, the process of forming a mandatory set of metadata for one of the collections of the digital library Lobachevskii-DML – a retro-collection of articles of the journal "Izvestia of the Physics and Mathematics Society at Kazan University" is presented.