From mboxrd@z Thu Jan 1 00:00:00 1970 X-Msuck: nntp://news.gmane.io/gmane.science.mathematics.categories/7416 Path: news.gmane.org!not-for-mail From: claudio pisani Newsgroups: gmane.science.mathematics.categories,gmane.spam.detected Subject: question on terminology Date: Fri, 24 Aug 2012 20:03:59 +0100 (BST) Message-ID: Reply-To: claudio pisani NNTP-Posting-Host: plane.gmane.org Mime-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Trace: ger.gmane.org 1345853278 26184 80.91.229.3 (25 Aug 2012 00:07:58 GMT) X-Complaints-To: usenet@ger.gmane.org NNTP-Posting-Date: Sat, 25 Aug 2012 00:07:58 +0000 (UTC) To: categories@mta.ca Original-X-From: majordomo@mlist.mta.ca Sat Aug 25 02:07:55 2012 Return-path: Envelope-to: gsmc-categories@m.gmane.org Original-Received: from smtpy.mta.ca ([138.73.1.128]) by plane.gmane.org with esmtp (Exim 4.69) (envelope-from ) id 1T53ut-0006f8-48 for gsmc-categories@m.gmane.org; Sat, 25 Aug 2012 02:07:55 +0200 Original-Received: from mlist.mta.ca ([138.73.1.63]:56269) by smtpy.mta.ca with esmtp (Exim 4.77) (envelope-from ) id 1T53tb-0003U0-4M; Fri, 24 Aug 2012 21:06:35 -0300 Original-Received: from majordomo by mlist.mta.ca with local (Exim 4.71) (envelope-from ) id 1T53tk-0004zl-Tw for categories-list@mlist.mta.ca; Fri, 24 Aug 2012 21:06:44 -0300 Precedence: bulk X-Spam-Report: 5.7 points; * 1.4 DATE_IN_PAST_03_06 Date: is 3 to 6 hours before Received: date * 2.5 LOCALPART_IN_SUBJECT Local part of To: address appears in Subject * 1.8 MIME_QP_LONG_LINE RAW: Quoted-printable line longer than 76 chars Xref: news.gmane.org gmane.science.mathematics.categories:7416 gmane.spam.detected:5001456 Archived-At: Dear categorists,=0A=0AIs there a standard name for those presheaves X on a= category C such that Xf is a bijection for any f in C?=0AI have sometimes = called them "biactions" since any such X (considered as, say, a left action= of C) is paired with the obvious presheaf X' on C^op (a right action of C)= : X'f =3D (Xf)^-1. =0AOf course, they correspond, as categories over C, to = discrete bifibrations.=0AI also know that the separable or decidable preshe= aves are those for which every Xf is injective.=0A=0AClaudio [For admin and other information see: http://www.mta.ca/~cat-dist/ ]