math.NT Number Theory
- From Number Theory comes the ideas and theoretical basis for modern cryptography, used to secure communications everywhere from banking to cellphones.
- A more quirky one is SETI (the primes in binary would be a very clear indication of a signal non-natural origin, and would be a starting point for communication).
- There is a gamma ray telescope design using mod p quadratic residues to construct a mask. Gamma rays cannot be focused, so this design uses a redundant array of detectors separated from the mask to reconstruct directional information.