I always think of number theory when the idea of uselessly abstract knowledge comes up - it was the purest of pure math for centuries and it was absurd to sugge

I always think of number theory when the idea of uselessly abstract knowledge comes up - it was the purest of pure math for centuries and it was absurd to suggest that things like the nature of prime numbers and the divisibility of integers would somehow be applied in industry. Then computers and cryptography came around and now there's a 2 trillion dollar ecommerce industry that depends on prime factorization being hard so people don't steal your bank information.