References
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- illustrating mathematical connection;A Geometric Proof of Euler's theorem no.봄 박경하, 이성원
- American Math. Monthly v.103 The Euler Characteristic and Ploya's Dream P. Hilton;J. Pedersen
- Lecture notes in Mathematics v.1000 Differenatical Geometry in the large H. hopf
- Algebraic Topology;An Introduction W. S. Massey
- Mathematical Discovery G. Polya