▎ 摘　　要

The energy density of an elastic sheet is the sum of a mean curvature H term, 2k(c)H(2), and a Gaussian curvature K term (k) over barK, where k(c), (k) over bar are constants. The sign of k(c) is commonly understood to be positive, but the sign of (k) over bar and even the role of K are subject to debate. We give reasons for the Gaussian contribution to be (k) over bar vertical bar K vertical bar instead, with (k) over bar positive. This is used to obtain the mean amplitude of thermal fluctuations of a graphene sheet. (C) 2011 Elsevier B.V. All rights reserved.