## 2 Definition of Feature Groups  
The attribution tasks and the definition of the author's style are similar in many ways; they require the determination of the style elements inherent in a particular author, but the attribution tasks are often formulated somewhat differently, for example as described in [7]. Within the framework of this work, the classification serves as the basis for the analysis of features in order to highlight the characteristics of the author's style. From the point of view of philology, the individual (written) style is a complex concept reflecting the socio-historical nature, ethnic, psychological, moral, and ethical characteristics of the author [8]. The various authors identify the different levels of text analysis [8; 9] that can be used in stylometry and, upon combining them, we find the following set of levels:  
* The phonetic level takes into account the peculiarity of intonation and melodica. The melodica describes the number of syllables in words, the repetitions of vowels and consonants to enhance expressive power and euphony. In poetic works, the melodica is aimed at ensuring the rhythm and the harmony. * The punctuation level reveals the peculiarities of the author's usage of punctuation marks and characteristic errors. * The spelling level reveals the characteristic errors in the spelling of words. * The lexical level describes the author's vocabulary, the features of the usage of words, phrases and stable expressions, as well as typical repetitive parts of sentences; a tendency to use rare and foreign words, synonyms, antonyms, paronyms, neologisms, as well as words denoting certain concepts. * The syntactic level describes the features of sentence construction, the prevailing types of sentences (affirmative, interrogative, exclamation), the tendency to use complex or incomplete, as well as elliptical sentences, types of syntactic coherence, word order, and sentence length. * The stylistic level is responsible for the genre, plot, and general structure of the text, as well as characteristic artistic means. As only published texts are considered in this work, the presence of spelling and punctuation errors is excluded. In the work [9], it is noted that the author's style usually refers to the features of the text at the highest levels, namely syntactic, lexical, and stylistic levels. It should be noted that these levels are widely used in expert analysis, but for formal computational analysis, these groups are quite complex, so most researchers use phonetic, as well as punctuation, lexical, and syntactic levels for analysis. Within the framework of this work, the groups of features are identified that actively influence the definition of authorship and characterize the author's style the most. The following groups of features were identified:  
* the distribution by parts of speech and relationships;
* the punctuation mark frequencies;
* the words and their n-grams;
* the service words;
* the letters and other symbols, as well as their n-grams;
* the metrorhythmic features. The distribution of words by parts of speech and relations corresponds to the syntactic level; the frequency of punctuation marks corresponds to the punctuation level. The features describing the words and their n-grams, as well as service words, are related to the lexical level. The features describing the distribution by letters and other symbols, as well as their n-grams, largely reflect the melodica of the text and describe the text at the phonetic level. In addition, the author can consciously control such a distribution to a lesser extent, unlike, for example, the words used. The metrorhythmic features can also be attributed to the phonetic level. Thus, all levels of text description are used in the study, with the exception of stylistic, but it is important to note that the description of the text using the selected set of features is not exhaustive at any level, so further work can be aimed at building the new effective features to improve the quality of text description at various levels.