<p>A graph <i>G</i>(<i>V</i>,&#xa0;<i>E</i>) is <i>word-representable</i>, if there exists a word <i>w</i> over the alphabet <i>V</i> such that for any two distinct letters <i>x</i> and <i>y</i>, <InlineEquation ID="IEq1"> <EquationSource Format="TEX">\(\{x,y\}\in V\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mo stretchy="false">{</mo> <mi>x</mi> <mo>,</mo> <mi>y</mi> <mo stretchy="false">}</mo> <mo>∈</mo> <mi>V</mi> </mrow> </math></EquationSource> </InlineEquation>, <i>x</i> and <i>y</i> alternate in <i>w</i> if and only if <InlineEquation ID="IEq2"> <EquationSource Format="TEX">\(xy \in E\)</EquationSource> <EquationSource Format="MATHML"><math> <mrow> <mi>x</mi> <mi>y</mi> <mo>∈</mo> <mi>E</mi> </mrow> </math></EquationSource> </InlineEquation>. In this paper, we prove that every non-empty word-representable graph can be represented by a word containing no non-trivial squares. This result provides a positive answer to the open problem present in the book <i>Words and graphs</i> written by <i>Sergey Kitaev</i>, and <i>Vadim Lozin</i>. We further prove that, for a word-representable graph <i>G</i> with representation number <i>k</i>, every <i>k</i>-uniform word representing <i>G</i> is also square-free. We also prove that every minimal-length word representing a graph is square-free. Moreover, we count the number of possible square-free word-representations of a complete graph. Then, we provide an example of a non-complete word-representable graph which has a finite number of square-free word-representations. Finally, using the infinite square-free string generated from the Thue-Morse sequence, we prove that there exist infinitely many square-free word-representants for these remaining non-complete connected word-representable graphs.</p>

错误:搜索内容不能为空,请输入英文关键词
错误:关键词超出字数限制,请精简
高级检索

Square-free Word-representation of Word-representable Graphs

  • Biswajit Das,
  • Ramesh Hariharasubramanian

摘要

A graph G(VE) is word-representable, if there exists a word w over the alphabet V such that for any two distinct letters x and y, \(\{x,y\}\in V\) { x , y } V , x and y alternate in w if and only if \(xy \in E\) x y E . In this paper, we prove that every non-empty word-representable graph can be represented by a word containing no non-trivial squares. This result provides a positive answer to the open problem present in the book Words and graphs written by Sergey Kitaev, and Vadim Lozin. We further prove that, for a word-representable graph G with representation number k, every k-uniform word representing G is also square-free. We also prove that every minimal-length word representing a graph is square-free. Moreover, we count the number of possible square-free word-representations of a complete graph. Then, we provide an example of a non-complete word-representable graph which has a finite number of square-free word-representations. Finally, using the infinite square-free string generated from the Thue-Morse sequence, we prove that there exist infinitely many square-free word-representants for these remaining non-complete connected word-representable graphs.