I'm writing a report on the philosophy of pure mathematical knowledge, and how pure mathematics isn't what people commonly consider it, as non-applied. I want to see if anyone has an example of, or can make up a simple mathematical proof/theorem/example, in the realm of pure mathematics, that actually has an application, although one that cannot be seen a first sight.
I want this to be a brief thing that I can add in the report and explain,
I appreciate your help,