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This paper reviews recent progress in understanding phenomena such as crumpling, in which elastic membranes or sheets subject to structureless forces develop sharply curved structure over a small fraction of their surface. In the limit of zero thickness, these structures become singular. After reviewing several related phenomena, the paper recalls the physical elements that give rise to the singular behavior: elasticity and the nearly inextensible behavior of thin sheets. This singular behavior has counterparts in higher dimensions. Then the paper discusses the most basic of these singularities, the vertex. The paper recounts mathematical progress in describing the d-cone, a simple realization of a vertex. After discussing the size of the core that governs departure from singularity, the paper concludes that fundamental understanding is lacking. It points out further mysterious behavior at the region where a d-cone is supported. Next comes a discussion of an emergent singularity that appears when two or more vertices are present: the stretching ridge. The paper offers several explanations of the scale of this singularity, ranging from qualitative scaling arguments to a formal asymptotic analysis. It discusses recent experiments and theories about the interaction of ridges and vertices and reviews evidence that these ridges dominate the mechanics of crumpled sheets. [ABSTRACT FROM AUTHOR]

The goal of this work is to introduce a coherent theory of the counterintuitive phenomena of dynamical destabilization under the action of dissipation. While the existence of one class of dissipation-induced instabilities was known to Sir Thomson (Lord Kelvin), it was not realized until recently that there is another major type of these phenomena hinted at by one of Merkin's theorems; in fact, these two cases exhaust all the generic possibilities. The theory grounded on the Thomson-Tait-Chetayev and Merkin theorems and on the geometric understanding introduced in this paper leads to the conclusion that ubiquitous dissipation is one of the paramount mechanisms by which instabilities develop in nature. Along with a historical review, the main theoretical achievements are put in a general context, thus unifying the current knowledge in this area and the multitude of relevant physical problems scattered over a vast literature. This general view also highlights the striking connection to various areas of mathematics. To appeal to the reader's intuition and experience, a large number of motivating examples are provided. The paper contains some new unpublished results and insights, and, finally, open questions are formulated to provide an impetus for future studies. While this review focuses on the finite-dimensional case, where the theory is relatively complete, a brief discussion of the current state of knowledge in the infinite-dimensional case, typified by partial differential equations, is also given. [ABSTRACT FROM AUTHOR]