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
Line graph of a hypergraph graph.). In hypergraph theory, it is often natural to require that hypergraphs be "k"-uniform. Every graph is the line graph of some hypergraph, but, given a fixed edge size "k", not every graph is a line graph of some "k"-uniform hypergraph. A main problem is to characterize those that are, for each "k" ≥ 3. A hypergraph is linear if each pair of hyperedges intersects in at most one vertex. Every graph is the line graph, not only of some hypergraph, but of some linear hypergraph . characterized line graphs of graphs by a list of 9 forbidden induced subgraphs.