Divisibility is the foundational concept of number theory: a divides b (written a|b) means b = ka for some integer k. Divisibility is reflexive and transitive, establishing a partial order on integers. Rigorous treatment develops divisibility properties essential for all subsequent number-theoretic structures.
Start with concrete examples (12|48, 7∤50) and verify the formal definition. Prove basic properties: if a|b and b|c, then a|c.
Confusing divisibility with being divisible (a|b means b is divisible by a). Thinking divisibility requires positive integers (it applies to all nonzero integers).
You already know from the Fundamental Theorem of Arithmetic that every integer greater than 1 factors uniquely into primes. Divisibility theory is the formal scaffolding beneath that theorem — it gives precise meaning to "a divides b" and establishes the structural properties that make the rest of number theory work. The definition is deceptively simple: we say a divides b (written a | b) if there exists an integer k such that b = k · a. So 4 | 12 because 12 = 3 · 4, and 7 ∤ 50 because no integer k satisfies 50 = 7k. Notice the notation: a | b says "a divides b," which is equivalent to saying "b is divisible by a." The two phrasings are converses of each other, and confusing them is a persistent error.
The formal treatment establishes that divisibility is a partial order on the positive integers — it is reflexive (a | a, since a = 1 · a), antisymmetric (if a | b and b | a with positive integers, then a = b), and transitive (if a | b and b | c, then a | c). The transitivity proof is where mathematical induction connects: the proof is direct, not inductive, but induction is the engine for more complex divisibility arguments that follow. The key properties to internalize are: if a | b and a | c, then a | (bx + cy) for any integers x, y — that is, divisibility is preserved under linear combinations. This is the seed from which Bézout's identity and the theory of greatest common divisors grow.
What makes the formal treatment powerful is that it extends cleanly to negative integers. We allow a and b to be any nonzero integers: −4 | 12 because 12 = (−3)(−4). This generality is essential for working in the integers as a ring, where negative numbers play equal status with positive ones. The Fundamental Theorem tells you *what* the prime factors are; divisibility theory tells you *how divisibility behaves structurally* — the rules of the game. Every subsequent result, from Bézout's identity to linear Diophantine equations, is built on exactly these properties.
The deepest insight at this stage is that divisibility gives the integers a lattice structure: for any two positive integers a and b, there is a unique greatest common divisor gcd(a,b) and a unique least common multiple lcm(a,b), related by gcd(a,b) · lcm(a,b) = ab. The gcd is the largest element below both a and b in the divisibility partial order; the lcm is the smallest element above both. This perspective reframes gcd and lcm as structural features of the integer lattice, not just computational results — a viewpoint that pays dividends when you move to more abstract algebraic settings.