Ebook A pythagorean introduction to number theory: Right triangles, sums of squares, and arithmetic - Part 2

Continued part 1, part 2 of ebook "A pythagorean introduction to number theory: Right triangles, sums of squares, and arithmetic" has presents the following content: advanced topics; counting pythagorean triples modulo an integer; how many lattice points are there on a circle or a sphere; what about geometry; another proof of the four squares theorem; quadratic forms and sums of squares; how many pythagorean triples are there; how are rational points distributed, really;...