Massachusetts Institute of Technology: Srini Devadas and Eric Lehman's "Number Theory I"
Read Section 1, "Divisibility" on pages 1 - 4 and Section 4, "The Greatest Common Divisor" on pages 7 - 12. This reading overlaps that of Bender and Williamson.
Click https://s3.amazonaws.com/saylordotorg-resources/wwwresources/site/wp-content/uploads/2011/09/CS202-NumberTheory1.pdf link to open resource.