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| '''Linguistic sequence complexity''' (LC) is a measure of the 'vocabulary richness' of a text.<ref name=Trifonov1990>{{cite book| author=[[Edward N. Trifonov]] |year=1990| book=Structure & Methods| title=Structure and Methods| series= Human Genome Initiative and DNA Recombination| volume=1| pages=69–77|chapter=Making sense of the human genome|publisher=Adenine Press, New York}}</ref>
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| When a [[nucleotide]] sequence is written as text using a four-letter alphabet, the repetitiveness of the text, that is, the repetition of its [[N-gram|N-grams (words)]], can be calculated and serves as a measure of sequence complexity. Thus, the more complex a [[DNA sequence]], the richer its [[oligonucleotide]] vocabulary, whereas repetitious sequences have relatively lower complexities. Subsequent work improved the original algorithm described in ([[Edward Trifonov|Trifonov]] 1990)<ref name=Trifonov1990/> without changing the essence of the linguistic complexity approach.<ref name=Gabrielian1999>{{cite doi|10.1016/S0097-8485(99)00007-8|noedit}}</ref><ref name=Orlov2004>{{cite doi|10.1093/nar/gkh466|noedit}}</ref><ref name=Janson2004>{{cite doi|10.1016/j.tcs.2004.06.023|noedit}}</ref>
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| The meaning of LC may be better understood by regarding the presentation of a sequence as a [[Tree (data structure)|tree]] of all subsequences of the given sequence. The most complex sequences have maximally balanced trees, while the measure of imbalance or tree asymmetry serves as a complexity measure. The number of nodes at the tree level {{math|<var>i</var>}} is equal to the actual vocabulary size of words with the length {{math|<var>i</var>}} in a given sequence; the number of nodes in the most balanced tree, which corresponds to the most complex sequence of length N, at the tree level {{math|<var>i</var>}} is either 4<sup>i</sup> or N-j+1, whichever is smaller. Complexity ({{math|<var>C</var>}}) of a sequence fragment (with a length RW) can be directly calculated as the product of vocabulary-usage measures (U<sub>i</sub>):<ref name=Gabrielian1999 />
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| <math>C = U_1 U_2...U_i....U_w </math>
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| Vocabulary usage for [[oligomers]] of a given size {{math|<var>i</var>}} can be defined as the ratio of the actual vocabulary size of a given sequence to the maximal possible vocabulary size for a sequence of that length. For example, U<sub>2</sub> for the sequence ACGGGAAGCTGATTCCA = 14/16, as it contains 14 of 16 possible different dinucleotides; U<sub>3</sub> for the same sequence = 15/15, and U<sub>4</sub>=14/14. For the sequence ACACACACACACACACA, U<sub>1</sub>=1/2; U<sub>2</sub>=2/16=0.125, as it has a simple vocabulary of only two dinucleotides; U<sub>3</sub> for this sequence = 2/15. k-tuples with k from two to W considered, while W depends on RW. For RW values less than 18, W is equal to 3; for RW less than 67, W is equal to 4; for RW<260, W=5; for RW<1029, W=6, and so on. The value of {{math|<var>C</var>}} provides a measure of sequence complexity in the range 0<C<1 for various DNA sequence fragments of a given length.<ref name=Gabrielian1999 />
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| This formula is different from the original LC measure<ref name=Trifonov1990/> in two respects: in the way vocabulary usage U<sub>i</sub> is calculated, and because {{math|<var>i</var>}} is not in the range of 2 to N-1 but only up to W. This limitation on the range of U<sub>i</sub> makes the algorithm substantially more efficient without loss of power.<ref name=Gabrielian1999 />
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| This sequence analysis complexity calculation can be used to search for conserved regions between compared sequences for the detection of low-complexity regions including simple sequence repeats, imperfect [[Direct repeat|direct]] or [[inverted repeat]]s, polypurine and polypyrimidine [[Triple-stranded DNA|triple-stranded DNA structures]], and four-stranded structures (such as [[G-quadruplex]]es).<ref name=Kalendar2011>{{cite doi|10.1016/j.ygeno.2011.04.009|noedit}}</ref>
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| == References ==
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| {{reflist}}
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| [[Category:Nucleic acids]]
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| [[Category:Bioinformatics]]
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