For competitive programming
- https://oeis.org
- https://www.wolframalpha.com
- https://visualgo.net/en
- https://wwwhomes.uni-bielefeld.de/achim/highly.txt (danh sách một vài số có nhiều ước)
- https://csacademy.com/app/graph_editor/
- https://csacademy.com/app/geometry_widget/