77

77 is a natural number that follows 76 and precedes 78. In base-10 notation it is written as two sevens, making it a repdigit. It has factors 1, 7, 11, and 77, with prime factorization 7 × 11.

In number theory, 77 is a semiprime and square-free. Its Euler’s totient value is φ(77) = (7−1)(11−1) = 60, and its sum of divisors is σ(77) = 96.

In science, the atomic number 77 corresponds to iridium (Ir), a dense, corrosion-resistant metal of the platinum group. Iridium is used in high-performance alloys, catalytic applications, and some electrical contacts due to its stability and hardness.

In computing, the decimal value 77 corresponds to the ASCII character 'M' (hex 0x4D). The Roman numeral representation of 77 is LXXVII.

Culturally, 77 is often encountered as a recognizable double-digit number in puzzles, branding, and numerology, where its symmetry and repetition contribute to its memorability.