Planar graphs without cycles of length from 4 to 7 are proved to be 3-colorable. Moreover, it is proved that each proper 3-coloring of a face of length from 8 to 11 in a connected plane graph without cycles of length from 4 to 7 can be extended to a proper 3-coloring of the whole graph. This improves on the previous results on a long standing conjecture of Steinberg.
The other theorem states that planar graphs without cycles of length from 4 to 7 are 3-colorable (see REF ).
Planar graphs without cycles of length from 4 to 7 are 3-colorable
J. Comb. Theory, Ser. B
J. Comb. Theory, Ser. B
Mathematics Computer Science