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2011 The entropy efficiency of point-push mapping classes on the punctured disk
Philip Boyland, Jason Harrington
Algebr. Geom. Topol. 11(4): 2265-2296 (2011). DOI: 10.2140/agt.2011.11.2265


We study the maximal entropy per unit generator of point-push mapping classes on the punctured disk. Our work is motivated by fluid mixing by rods in a planar domain. If a single rod moves among N fixed obstacles, the resulting fluid diffeomorphism is in the point-push mapping class associated with the loop in π1(D2{N points}) traversed by the single stirrer. The collection of motions where each stirrer goes around a single obstacle generate the group of point-push mapping classes, and the entropy efficiency with respect to these generators gives a topological measure of the mixing per unit energy expenditure of the mapping class. We give lower and upper bounds for Eff(N), the maximal efficiency in the presence of N obstacles, and prove that Eff(N) log(3) as N. For the lower bound we compute the entropy efficiency of a specific point-push protocol, HSPN, which we conjecture achieves the maximum. The entropy computation uses the action on chains in a –covering space of the punctured disk which is designed for point-push protocols. For the upper bound we estimate the exponential growth rate of the action of the point-push mapping classes on the fundamental group of the punctured disk using a collection of incidence matrices and then computing the generalized spectral radius of the collection.


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Philip Boyland. Jason Harrington. "The entropy efficiency of point-push mapping classes on the punctured disk." Algebr. Geom. Topol. 11 (4) 2265 - 2296, 2011.


Received: 8 April 2011; Revised: 23 July 2011; Accepted: 25 July 2011; Published: 2011
First available in Project Euclid: 19 December 2017

zbMATH: 1243.37036
MathSciNet: MR2826939
Digital Object Identifier: 10.2140/agt.2011.11.2265

Primary: 37E30

Rights: Copyright © 2011 Mathematical Sciences Publishers


Vol.11 • No. 4 • 2011
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