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Colour refinement algorithm
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<p>In <a href="/facts/Graph_theory/VVPCd4TX">graph theory</a> and <a href="/facts/Theoretical_computer_science/jSkGGuvy">theoretical computer science</a>, the colour refinement algorithm also known as the naive vertex classification, or the 1-dimensional version of the <a href="/facts/Weisfeiler_Leman_graph_isomorphism_test/W1NIeftl">Weisfeiler-Leman algorithm</a>, is a routine used for testing whether two graphs are <a href="/facts/Isomorphic_graph/SBjVIhwW">isomorphic</a>. While it solves graph isomorphism on almost all graphs, there are graphs such as all regular graphs that cannot be distinguished using colour refinement. </p>