A Lemmatization Method for Modern Mongolian and its Application to Information Retrieval

A Lemmatization Method for Modern Mongolian and its Application to Information Retrieval Badam-Osor Khaltar Atsushi Fujii Graduate School of Library, Information and Media Studies University of Tsukuba 1-2 Kasuga Tsukuba, 305-8550, Japan {khab23, fujii}@slis.tsukuba.ac.jp Abstract In Modern Mongolian, a content word can be inflected when concatenated with suf- fixes. Identifying the original forms of content words is crucial for natural lan- guage processing and information retrieval. We propose a lemmatization method for Modern Mongolian and apply our method to indexing for information retrieval. We use technical abstracts to show the effec- tiveness of our method experimentally. 1 Introduction The Mongolian language is divided into Tradition- al Mongolian, which uses the Mongolian alphabet, and Modern Mongolian, which uses the Cyrillic alphabet. In this paper, we focus solely on the lat- ter and use the word “Mongolian” to refer to Mod- ern Mongolian. In Mongolian, which is an agglutinative lan- guage, each sentence is segmented on a phrase-by- phrase basis. A phrase consists of a content word, such as a noun or a verb, and one or more suffixes, such as postpositional participles. A content word can potentially be inflected when concatenated with suffixes. Identifying the original forms of content words in Mongolian text is crucial for natural language processing and information retrieval. In informa- tion retrieval, the process of normalizing index terms is important, and can be divided into lemma- tization and stemming. Lemmatization identifies the original form of an inflected word, whereas stemming identifies a stem, which is not necessari- ly a word. Existing search engines, such as Google and Yahoo!, do not perform lemmatization or stem- ming for indexing Web pages in Mongolian. Therefore, Web pages that include only inflected forms of a query cannot be retrieved. In this paper, we propose a lemmatization me- thod for Mongolian and apply our method to in- dexing for information retrieval. 2 Inflection types in Mongolian phrases Nouns, adjectives, numerals, and verbs can be concatenated with suffixes. Nouns and adjectives are usually concatenated with a sequence of a plural suffix, case suffix, and reflexive possessive suffix. Numerals are concatenated with either a case suffix or a reflexive possessive suffix. Verbs are concatenated with various suffixes, such as an aspect suffix, a participle suffix, and a mood suffix. Figure 1 shows the inflection types of content words in Mongolian phrases. In (a), there is no in- flection in the content word “ном (book)”, conca- tenated with the suffix “ын (the genitive case)”. The content words are inflected in (b)-(e). Type Example (a) No inflection ном + ын → номын book + genitive case (b) Vowel insertion ах + д → ахад brother + dative case (c) Consonant insertion байшин + ийн→ байшингийн building + genitive case (d) The letters “ь” or “и” are eliminated, and the vowel converts to “и” анги + аас → ангиас return + ablative case (e) Vowel elimination ажил + аас → ажлаас work + ablative case Figure 1: Inflection types of content words in Mongolian phrases. 1 Loanwords, …

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