Classification of documents with the assignment of classifier codes is a traditional way of systematizing and searching for documents on a specific topic. The Universal Decimal Classification (UDC) underlies the systematization of knowledge presented in libraries, databases and other information repositories. In Russia, UDC is an obligatory attribute of all book production and information on natural and technical sciences. The choice of classification codes is associated with the analysis of the structure of the classifier tree and is traditionally decided by the author of a scientific article. This article proposes a solution for automating the assigning the UDC classification code for a mathematical article based on a special resource – the OntoMath^PRO ontology for professional mathematics, developed at Kazan Federal University. An approach to solving the problem is to create "code maps" for each classifying code in the UDC tree in the field of mathematics. Under the "code map" is meant a weighted set of all extracted, with the help of OntoMath^PRO ontology, mathematical named entities from the collection of articles with a given UDC code. The creation of "code maps" is based on the hypothesis that the choice of the UDC code is determined by a certain set of classifying features that can be represented by classes from the OntoMath^PRO ontology. The proposed hypothesis was tested and confirmed in the paper. The hypothesis was tested on a collection of mathematical articles An approach to solving the problem is to create "code maps" for each classifying code in the UDC tree in the field of mathematics. Under the "code map" is meant a weighted set of all extracted, with the help of OntoMath^PRO ontology, mathematical named entities from the collection of articles with a given UDC code. The creation of "code maps" is based on the hypothesis that the choice of the UDC code is determined by a certain set of classifying features that can be represented by classes from the OntoMath^PRO ontology. The proposed hypothesis was tested and confirmed in the paper.  The hypothesis was tested on a collection of mathematical articles published during 1999-2009 in the "Izvestiya VUZov. Mathematics" journal.