= Wave packet
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A <wave packet> is a localized <superposition> of <Fourier modes>. In a narrow packet, nearby <wavenumbers> interfere to produce a slowly varying <wave envelope> around a rapid <carrier wave>. Its leading translation is governed by <group velocity>; the carrier's <wave phase> follows <phase velocity>. Localized does not necessarily mean <compact support>: a <Gaussian function> decays rapidly but extends over the whole line. The <Gaussian envelope in a Klein-Gordon medium> gives an original example with both translation and spreading.
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