Finitely generated monoids are finitely presented? - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-24T15:14:33Z http://mathoverflow.net/feeds/question/36623 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/36623/finitely-generated-monoids-are-finitely-presented Finitely generated monoids are finitely presented? Yuhao Huang 2010-08-25T04:27:14Z 2013-05-20T12:21:32Z <p>I saw in the answer of <a href="http://mathoverflow.net/questions/28817/structure-theorem-for-finitely-generated-commutative-cancellative-monoids" rel="nofollow">this post</a> that any finitely generated monoids are finitely presented in the sense that there is a coequalizer diagram $P_1\rightrightarrows P_0\rightarrow M$ with $P_1$ and $P_0$ free commutative and finitely generated.</p> <p>My question is:</p> <p>Can we make $P_1$ maps to the kernel of $P_0\rightarrow M$? (to be more, precise, can we find a presentation of the form $P_1\rightarrow P_0\rightarrow M$ like the case for abelian groups?)</p> <p>And if a monoid is finitely presented for one presentation, is it the same for other presentations? Thanks</p> http://mathoverflow.net/questions/36623/finitely-generated-monoids-are-finitely-presented/38255#38255 Answer by Mark Sapir for Finitely generated monoids are finitely presented? Mark Sapir 2010-09-10T01:13:19Z 2010-09-10T01:13:19Z <p>Every finitely generated commutative monoid is indeed finitely presented. Of course the presentation depends on the generating set. But the minimal presentation will be always finite (provided you choose finite generating sets). See <a href="http://www.math.vanderbilt.edu/~msapir/ftp/pub/survey/survey.pdf" rel="nofollow">http://www.math.vanderbilt.edu/~msapir/ftp/pub/survey/survey.pdf</a> </p> http://mathoverflow.net/questions/36623/finitely-generated-monoids-are-finitely-presented/131177#131177 Answer by John Baez for Finitely generated monoids are finitely presented? John Baez 2013-05-20T01:00:15Z 2013-05-20T01:00:15Z <p>Nobody has answered the actual question, though of course the preface to the question should say "any finitely generated <em>commutative</em> monoid is finitely presented in the sense that there is a coequalizer diagram $P_1 \stackrel{\to}{\to} P_0 \to M$ with $P_1$ and $P_0$ free commutative and finitely generated."</p> <p>I believe the answer to the question is <em>no</em>, we cannot always find a presentation $P_1 \to P_0 \to M$ where $P_1$ maps to the kernel of $P_0 \to M$. </p> <p>However, I don't have a proof.</p> http://mathoverflow.net/questions/36623/finitely-generated-monoids-are-finitely-presented/131214#131214 Answer by Benjamin Steinberg for Finitely generated monoids are finitely presented? Benjamin Steinberg 2013-05-20T12:21:32Z 2013-05-20T12:21:32Z <p>The answer is no if the OP means all relations should be consequences of the relations of the form w=1 with 1 the identity. Let S be the multiplicative monoid of the two-element field. If F is a free commutative monoid with generators A and if F maps onto S via f, then if we set B to be the generators mapping to 1 under f, we have a word represents the identity iff it is a product of letters from B. But if I add all such relations I will get as the quotient a free commutative monoid on A-B not S. </p>