Step 1 of 4 · The definition
Every whole number ≥ 2 is either prime or composite.
PRIME = exactly two factors (1 and itself).
COMPOSITE = more than two factors.
(1 is neither — it has only one factor.)
To test if N is prime: try dividing by every prime ≤ √N. If any divide evenly, N is composite. Otherwise prime.
Step 2 of 4 · Is 23 prime?
23
√23 ≈ 4.8, so we only need to test primes 2 and 3.
PRIME — no primes ≤ √23 divide 23