131

131 is a natural number that comes after 130 and before 132. In decimal notation it is a palindromic prime, meaning its digits form the same sequence forward and backward and it has no positive divisors other than 1 and itself. The number is odd, and the sum of its digits is 5. It is a Sophie Germain prime, since 2×131 + 1 = 263 is also prime. It is congruent to 3 modulo 4, so it cannot be expressed as a sum of two squares.

In different numeral systems, 131 has the following representations: Roman numerals, CXXXI; binary, 10000011; octal, 203; and hexadecimal, 83.

131 also appears in calendar and geography contexts. In the Roman calendar, 131 AD falls during the reign of Emperor Hadrian (117–138 AD). The year 131 BC would have occurred during the late Roman Republic. In modern infrastructure, U.S. Route 131 is a north–south highway in the U.S. state of Michigan that runs through western and central parts of the state.

As a prime, 131 participates in various mathematical discussions and can serve as an example of a palindromic prime and a Sophie Germain prime. Its palindromic property also makes it notable in considerations of numeral symmetry in base-10 representations.