中国科学院数学与系统科学研究院期刊网
A NOTE ON THE 4-VALUED PROPOSITIONAL CALCULUS AND THE FOUR COLOUR PROBLEM
SHIH-HUA HOO(1);CHIANG-YE
Acta Mathematica Sinica, Chinese Series . 1951, (3): 243 -246 .  DOI: 10.12386/A1951sxxb0011