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November 26, 2013; 110 (48)

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Cover image: Pictured is a part of a geometric model space for a mathematical group, one of the so-called Baumslag-Solitar groups. The areas of curves within this space can be measured by two different methods, called homological and homotopical Dehn functions. In the group pictured here, both functions grow exponentially; the red curve is one of a sequence of curves demonstrating this growth. Aaron Abrams, Noel Brady, Pallavi Dani, and Robert Young discovered finitely presented groups in which the two types of measurements produce different growth functions. See their article on pages 19206–19212, a part of the Special Feature on Quantitative Geometry. Image courtesy of Aaron Abrams, Jason Cantarella, and Tammy Cantarella.

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