Soft introduction to cohomology

Pythagorean triple and Hilbert theorem 90

http://www.math.harvard.edu/~elkies/Misc/hilbert.pdf

Isaksen, Daniel C. 2002. “A Cohomological Viewpoint on Elementary School Arithmetic”. The American Mathematical Monthly 109 (9) (11월 1): 796-805. doi:10.2307/3072368. http://www.jstor.org/stable/3072368

17 wallpaper groups