Hadwiger conjecture
... that mathematician Paul Erdős called the Hadwiger conjecture, a still-open generalization of the four-color problem, "one of the deepest unsolved problems in graph theory"?
... that <a href="./Mathematician" id="mwA30" rel="mw:WikiLink" title="Mathematician">mathematician</a> <a href="./Paul_Erdős" id="mwA34" rel="mw:WikiLink" title="Paul Erdős">Paul Erdős</a> called the <b id="mwA38"><a href="./Hadwiger_conjecture_(graph_theory)" id="mwA4A" rel="mw:WikiLink" title="Hadwiger conjecture (graph theory)">Hadwiger conjecture</a></b>, a still-open generalization of the <a class="mw-redirect" href="./Four-color_problem" id="mwA4E" rel="mw:WikiLink" title="Four-color problem">four-color problem</a>, "one of the deepest unsolved problems in <a href="./Graph_theory" id="mwA4I" rel="mw:WikiLink" title="Graph theory">graph theory</a>"?