Planck's law In 1880, André-Prosper-Paul Crova published a diagram of the three-dimensional appearance of the graph of the strength of thermal radiation as a function of wavelength and temperature. He determined the spectral variable by use of prisms. He analyzed the surface through what he called "isothermal" curves, sections for a single temperature, with a spectral variable on the abscissa and a power variable on the ordinate. He put smooth curves through his experimental data points. They had one peak at a spectral value characteristic for the temperature, and fell either side of it towards the horizontal axis. Such spectral sections are
Topological graph embeddings of graphs in surfaces (that is, drawings without crossings). A fundamental problem in extremal graph theory is the following: what is the maximum number of edges that a graph of "n" vertices can have if it contains no subgraph belonging to a given class of "forbidden subgraphs"? The prototype of such results is Turán's theorem, where there is one forbidden subgraph: a complete graph with "k" vertices ("k" is fixed). Analogous questions can be raised for topological and geometric graphs, with the difference that now certain "geometric subconfigurations" are forbidden. Historically, the first instance of such a theorem is