📜 WAYPOINT LESSON
Final 2-B: Coprimality Census
⭐ beginner⏳ 40 min read📍 Lesson 206 of 281
Final simulation, problem D: Mobius inclusion-exclusion over the value range. Target: solved within 40 minutes.
Final 2-B — Coprimality Census. gcd == 1 pairs: total = Sum mu(d) * C(c_d, 2). Graded near-miss: summing C(c_d, 2) for every d >= 2 without the Mobius signs.
Final 2-B. The wrong version triple-counts pairs like (6, 10, 15) whose gcds share several primes — the classic inclusion-exclusion failure.