TY  - BOOK
AU  - Ore Oystein
TI  - Four-colour problem
T2  - Pure and applied mathematics; 27
U1  - B44, K7 
PY  - 1977///
CY  - New York    
PB  -  Academic Press       
KW  - Grapth theory
N1  - Bibliography 249-254p
ER  -