What do $\Gamma$-sets classify? - MathOverflow most recent 30 from http://mathoverflow.net 2013-05-23T23:47:30Z http://mathoverflow.net/feeds/question/85600 http://www.creativecommons.org/licenses/by-nc/2.5/rdf http://mathoverflow.net/questions/85600/what-do-gamma-sets-classify What do $\Gamma$-sets classify? Mike Shulman 2012-01-13T17:35:21Z 2012-01-16T02:13:29Z <p>The category $\Gamma^{\mathrm{op}}$ is defined to be a skeleton of the category of finite pointed sets (see also <a href="http://mathoverflow.net/questions/74436/geometric-meaning-of-gamma-sets" rel="nofollow">this question</a>). Then $\Gamma$-spaces, meaning space-valued presheaves $\Gamma^{\mathrm{op}}\to \mathrm{Spaces}$, can be used to present spaces with commutative and associative multiplication up to all higher homotopies. This is similar to how the category $\Delta^{\mathrm{op}}$ can be defined as a skeleton of the category of finite total orders with distinct endpoints, and presheaves $\Delta^{\mathrm{op}}\to \mathrm{Spaces}$ (simplicial spaces) can be used to present spaces with associative multiplication up to all higher homotopies.</p> <p>It is known that the topos of simplicial sets is the classifying topos for total orders with distinct endpoints. Does the topos of set-valued presheaves on $\Gamma$ have a similar interpretation?</p> http://mathoverflow.net/questions/85600/what-do-gamma-sets-classify/85782#85782 Answer by Andreas Blass for What do $\Gamma$-sets classify? Andreas Blass 2012-01-16T02:13:29Z 2012-01-16T02:13:29Z <p>I agree with Charles Rezk's comment. Quite generally, the classifying topos for a universal Horn theory $T$ is the topos of covariant set-valued functors on the category of finitely presented models of $T$. This is proved (twice) in an old joint paper of mine and Andre Scedrov's, "Classifying topoi and finite forcing," J. Pure Appl. Algebra 28 (1983) 111-140. (For searching, it may help to know that the paper is so old that it uses the original form of Andre's name, "Andrej" and with \v accents on the S and c of his last name.) </p> <p>Apply that description of the classifying topos to the theory with no axioms in a vocabulary consisting of just one constant symbol, i.e., the theory of pointed sets. Its finitely presented models are just the finite pointed sets, so its classifying topos is the topos of covariant functors on the category $\Gamma^{\text{op}}$ of the question. Equivalently, it's the topos of pre sheaves on $\Gamma$. </p>