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(fer maz’) The seismic raypath between two points is that for which the first-order variation of traveltime with respect to neighboring paths is zero. It is sometimes phrased as that path for which the traveltime is a minimum (or, in certain cases, a maximum or a stationary value) compared with neighboring paths. If the intervening media have different speeds, the path will not be straight, but will be such that the over-all traveltime is minimized (usually). The resulting raypath is called the least-time path or brachistochrone. Snell’s law follows from Fermat’s principle. Named for Pierre Fermat (1601–1665), French mathematician.