John D. Cook (who has one of my favorite blogs on the web) recently had a few helpful posts about divisibility rules: - Divisibility by 7, 11, 13: https://www.

John D. Cook (who has one of my favorite blogs on the web) recently had a few helpful posts about divisibility rules: - Divisibility by 7, 11, 13: https://www.johndcook.com/blog/2020/11/10/test-for-divisibility-by-13/ https://www.johndcook.com/blog/2020/11/10/test-for-divisibil... - Divisibility by any prime using determinants: https://www.johndcook.com/blog/2021/02/17/divisibility-by-any-prime/ https://www.johndcook.com/blog/2021/02/17/divisibility-by-an...