In physics and optics, the Fraunhofer lines are a set of spectral lines named after the German physicist Joseph von Fraunhofer (1787–1826). The lines were originally observed as dark features (absorption lines) in the optical spectrum of the Sun. == Discovery == In 1802, the English chemist William Hyde Wollaston was the first person to note the appearance of a number of dark features in the solar spectrum. In 1814, Fraunhofer independently rediscovered the lines and began a systematic study and careful measurement of the wavelength of these features. In all, he mapped over 570 lines, and...

In mathematics, the notion of being compactly embedded expresses the idea that one set or space is "well contained" inside another. There are versions of this concept appropriate to general topology and functional analysis. == Definition (topological spaces) == Let (X, T) be a topological space, and let V and W be subsets of X. We say that V is compactly embedded in W, and write V ⊂⊂ W, if V ⊆ Cl(V) ⊆ Int(W), where Cl(V) denotes the closure of V, and Int(W) denotes the interior of W; and Cl(V) is compact. == Definition (normed spaces) == Let X and Y be two normed vector spaces with norms ||•||X...

In mathematics (specifically in measure theory), a Radon measure, named after Johann Radon, is a measure on the σ-algebra of Borel sets of a Hausdorff topological space X that is locally finite and inner regular. (Intuitively, it is useful in mathematical finance particularly for working with Lévy processes because it has the properties of both Lebesgue and Dirac measures, as unlike the Lebesgue, a Radon measure on a single point is not necessarily of measure 0.) == Motivation == A common problem is to find a good notion of a measure on a topological space that is compatible with the topology in...