Bert Randerath Ingo Schiermeyer 3-Colorability element oF P for p<sub>6</sub>-free Graphs. 85-89 2001 8 Electronic Notes in Discrete Mathematics http://dx.doi.org/10.1016/S1571-0653(05)80088-5 db/journals/endm/endm8.html#RanderathS01