Every even integer greater than 2 can be expressed as the sum of two primes — unproven since 1742
The Goldbach comet shows r(n) = the number of ways to write n as p+q for each even n. The Hardy-Littlewood conjecture predicts r(n) ~ 2C₂·n/(ln n)² where C₂ = Π (p-1)/(p-2) ≈ 0.6601 over odd primes p. Verified up to 4×10¹⁸.