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<p>(or point-set topology).</p>

<p><big> History </big></p>
<p>Around 1735, <a href="page.php?w=Leonhard_Euler">Leonhard Euler</a> discovered the <a href="page.php?w=Planar_graph">formula</a> <math>V - E + F = 2</math> relating the number of vertices (<math>V</math>), edges (<math>E</math>) and faces (<math>F</math>) of a <a href="page.php?w=Convex_polytope">convex polyhedron</a>, and hence of a <a href="page.php?w=planar_graph">planar graph</a>. The study and generalization of this formula, specifically by <a href="page.php?w=Augustin-Louis_Cauchy"> Cauchy</a></p><p>
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