121

121 is a natural number that follows 120 and precedes 122. In decimal notation, it is a three-digit palindrome and the square of the prime 11.

In mathematics, 121 is notable for being a square number and a palindrome. It equals 11^2, and its prime factorization is 11 × 11, making it a semiprime. It can be expressed as the sum 1 + 3 + 9 + 27 + 81, the geometric series that yields 121.

In base representations, 121 in base 10 is 1321 in base 4 and 11111 in base 3, since 1×4^3 + 3×4^2 + 2×4 + 1 = 121 and 1×3^4 + 1×3^3 + 1×3^2 + 1×3 + 1 = 121. In Roman numerals it is written as CXXI.

In the calendar, the 121st day of the year is May 1 in common years and April 30 in leap years.

Historically, the year 121 CE fell during the reign of Emperor Hadrian, a period noted for architectural activity in Rome and the consolidation of the empire.

See also: Palindromic numbers, Square numbers, Semiprimes.