De Bruijn–Erdős theorem
... that the De Bruijn–Erdős theorem may be used to extend the four-color theorem from finite planar graphs to planar graphs with infinitely many vertices?
... that the <b id="mwA_g"><a href="./De_Bruijn–Erdős_theorem_(graph_theory)" id="mwA_k" rel="mw:WikiLink" title="De Bruijn–Erdős theorem (graph theory)">De Bruijn–Erdős theorem</a></b> may be used to extend the <a href="./Four_color_theorem" id="mwA_o" rel="mw:WikiLink" title="Four color theorem">four-color theorem</a> from finite <a href="./Planar_graph" id="mwA_s" rel="mw:WikiLink" title="Planar graph">planar graphs</a> to planar graphs with infinitely many vertices?