Develop divisibility criteria for “nontraditional” primes, like 7 or 13. Can you explain why these are not usually mentioned in the regular literature?

Number Theory Survey of arithmetic functions σ, τ, µ, ϕ and how to compute them. Combinatorial proofs Understand divisibility criteria. Develop divisibility criteria for “nontraditional” primes, like 7 or 13. Can you explain why these are not usually mentioned in the regular literature?