https://math.uchicago.edu/~may/REU2015/REUPapers/Butt.pdf
UCLA The Gauss – Bonnet Theorem
https://www.math.ucla.edu/~greene/Gauss-Bonnet.htm
7. THE GAUSS-BONNET THEOREM – University of Pennsylvania
https://www2.math.upenn.edu/~shiydong/Math501X-7-GaussBonnet.pdf
The Gauss-Bonnet Theorem Denis Bell University of North Florida
https://www.unf.edu/~dbell/GBVB.pdf
Analysis Meets Topology: Gauss Bonnet Theorem
University of Utah
https://www.math.utah.edu/~treiberg/GaussBonnetSlides.pdf