121 is the smallest multidigit palindromic square. It is also the smallest palindromic composite such that some permutation of digits is prime. It is the only odd palindromic square with an odd number of digits that is divisible by a palindromic prime number with an even number of digits.
121 is a perfect square. It is also a centered octagonal number and a centered dodecagonal number.
Every number greater than 121 is the sum of distinct primes of the form 4n + 1.
121 is 11111 in base 3.
121 can be expressed as the sum of a prime and its reversal for exactly three distinct primes: 121 = 29 + 92 = 47 + 74 = 83 + 38.
121 = 1 + 31 + 32 + 33 + 34.
121 is the number of points required to win a standard game of cribbage.
Source: Prime Curios!