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

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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?