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# 3: Primes Numbers

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Primes are the atoms out of which the more complicated, composite integers (the molecules, in this metaphor) are built. In this chapter we study some of their basic properties, prove the aptly named Fundamental Theorem of Arithmetic, and go on to Wilson’s Theorem.

• 3.1: Basics and the FTA
Some primes are 2, 3, 5, 7, 11, 13, and 17. Notice 2 is the only even prime (clearly – any other would be a multiple of 2 and hence could not be prime), and has some unusual properties – the joke is that “ 2 is the oddest prime.” The largest prime known to humans at the time of this writing is (2^57,885,161) −1, which was proven to be prime in January of 2013 by a distributed computer program called GIMPS running on hundreds of machines across the Internet.
• 3.2: Wilson's Theorem
In this section, we prove a nice theorem usually named after an 18th century English mathematician ... although it was actually first stated by Ibn al-Haytham nearly 800 years earlier.
• 3.3: Multiplicative Order and Applications
In this section we prove two very useful results called Euler’s Theorem and Fermat’s Little Theorem (a special case of Euler’s). We do not follow the proof strategy of Euler and Fermat, however, instead using an approach inspired by abstract algebra and Lagrange’s Theorem in that subject.
• 3.4: Another Approach to Fermat's Little and Euler's Theorems
There is another way to think about these theorems which we shall explain here. In this case, we shall consider the theorems in the reverse order to what we used in previous sections – which is in fact more historically accurate.

Thumbnail: Sieve of Eratosthenes: algorithm steps for primes below $$121$$ (including optimization of starting from prime's square). (CC BY-SA 3.0; SKopp via Wikipedia)