Once thought of as the purest but least applicable part of mathematics, number theory is now by far the most commonly applied: every one of the millions of secure internet transmissions occurring each second is encrypted using ideas from number theory. This online high school Number Theory class covers the fundamentals of this classical, elegant, yet supremely relevant subject. This course provides a foundation for further study of number theory, but even more, it develops the skills of mathematical reasoning and proof in a concrete and intuitive way, great preparation for any future course in upper-level college mathematics or theoretical computer science. We progressively develop the tools needed to understand the RSA algorithm, the most common encryption scheme used worldwide. Along the way, we invent some encryption schemes of our own and discover how to play games using number theory. We also get a taste of the history of the subject, which involves the most famous mathematicians from antiquity to the present day, and we see parts of the story of Fermat’s Last Theorem, a 350-year-old statement that was fully proven only twenty years ago. While most calculations will be simple enough to do by hand, we will sometimes use the computer to see how the fundamental ideas can be applied to the huge numbers needed for modern applications. Course Prerequisite A strong background in precalculus and above, as well as a desire to do rigorous mathematics and proofs. GOA is offering 14 of its most popular semester classes in an intensive 7-week format. Most of these courses cover a semester's worth of material and expect a 10-12 hour per week commitment from students. For Geometry or Spanish 1, which are designed to replace yearlong high school courses, students should expect to dedicate 15-20 hours per week. Our summer term runs from June 15 to July 31, 2020.