Hierarchical structure (or hierarchy, simply) is a layer arrangement of objects, values, categories, etc. As with [faculty - department - laboratory] in universities and [department - division - section - unit] in companies, pyramid-type hierarchical structures are formed in human community. On the other hand, in nature, hierarchy emerges spontaneously with regard to spatial and temporal scale. For example, there exist [cell - protein - atom and molecule] in biology and [supergalaxy - galaxy cluster - galaxy - star] in universe.

　By focusing on a single scale, natural phenomena are approximately described by a linear law. The Hooke's law can represent the load-extension relation of springs; note that this linear law holds only in the case of sufficiently small load. However, many familiar phenomena consist of several scales that interact with each other. To describe such phenomena, nonlinear effect is essential.

　I've studied several nonlinear phenomena by theoretical analysis and numerical simulation based on the idea of statistical physics. A special interest is developed in why and how hierarchy in nature appears, and which functions emerge from the organization of hierarchy. As it is difficult to study universal features so far, I engage in several themes relevant to hierarchical structure; in future, I try to extract the universal understanding from those themes.