Totient

number theory studies the integers on their own terms — divisibility, primes, remainders — and for two millennia it was the purest of pure mathematics, prized precisely because it was useless. 𐃏 then in 1977 the RSA cryptosystem turned euclid, fermat and euler into load-bearing internet infrastructure. this page walks the classical results in dependency order and ends at that payoff (Epp, Susanna S., 2019).