= Leonhard Euler
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<Leonhard Euler> (1707–1783) developed <analysis> as a working language for <mechanics>, <geometry> and <number theory>. His 1736–1737 treatment of <Newtonian mechanics> put motion into the methods of <differential equations>, while his work on <surfaces> and <polyhedra> connected geometric quantities with structural relations such as the <Euler characteristic>.https://mathshistory.st-andrews.ac.uk/Biographies/Euler/[University of St Andrews, MacTutor biography, discussion of Mechanica and contributions to differential geometry and polyhedra]{ref}
The <Euler's formula> $e^{i\theta}=\cos\theta+i\sin\theta$ exemplifies the resulting unity: the <complex exponential> describes rotation, so oscillations can be manipulated through multiplication and differentiation. The <Euler characteristic> of a convex <polyhedron>, $V-E+F=2$, concerns a different invariant altogether. Distinguishing these meanings makes his many eponyms useful connections rather than interchangeable formulas.
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{title=Portrait of <Leonhard Euler>}
{description=Jakob Emanuel Handmann, 1753; Kunstmuseum Basel, inventory 276; reproduction cropped from Euler-2007 document. Public domain (life-plus-100, pre-1931 U.S. publication) and faithful reproduction / PD-Art. Source and usage information on the linked Wikimedia file page.}
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