GeeksforGeeks is the best to me, for any basic algorithm:

Mathematics: https://www.geeksforgeeks.org/mathematical-algorithms/

Geometry: https://www.geeksforgeeks.org/geometric-algorithms/

Game Theory: https://www.geeksforgeeks.org/category/algorithm/game-theory/

Bitwise: https://www.geeksforgeeks.org/bitwise-algorithms/

But the MathBlog is also well. link: http://www.mathblog.dk

https://forthright48.blogspot.com/

number theory

https://crypto.stanford.edu/pbc/notes/numbertheory/overview.html

Basic Number Theory : https://www.codechef.com/wiki/tutorial-number-theory/

Basic Geometry : https://www.topcoder.com/community/data-science/data-science-tutorials/geometry-concepts-basic-concepts/

Segment Intersection(Line): http://www.geeksforgeeks.org/check-if-two-given-line-segments-intersect/

Circle Intersection : http://mathworld.wolfram.com/Circle-CircleIntersection.html

Reflection & Rotation of Line: https://www.topcoder.com/community/data-science/data-science-tutorials/geometry-concepts-line-intersection-and-its-applications/

Line Sweep Algorithms: https://www.topcoder.com/community/data-science/data-science-tutorials/line-sweep-algorithms/

Over All Geometry : http://blog.informationgeometry.org/

cut-the-knot.org, AOPS(art of problem solving).

https://www.coolmathgames.com/