166

166 is a natural number that follows 165 and precedes 167. In mathematics, 166 is even and composite; its prime factorization is 2 × 83, making it a semiprime. It has four divisors: 1, 2, 83, and 166. Euler's totient function gives φ(166) = 82. In various numeration systems, 166 is written as CLXVI in Roman numerals, 10100110 in binary, and A6 in hexadecimal.

Calendar and dating: In day-of-year terms, the 166th day of a common year is June 15; in leap years it is June 14.

Historical context: The year 166 CE occurred during the reign of Marcus Aurelius and marked the beginning of the Marcomannic Wars against tribes along the Danube. The Antonine Plague, which has its origins around 165, affected the Roman Empire and continued to impact events in 166 and the years that followed.

Other notes: The number appears in various arithmetic and cataloging contexts but does not have a widely recognized special designation beyond its mathematical properties and historical usages.