129

129 is the sum of the first ten prime numbers. It is the smallest number that can be expressed as a sum of three squares in four different ways: 11 2 + 2 2 + 2 2 {\displaystyle 11^{2}+2^{2}+2^{2}}, 10 2 + 5 2 + 2 2 {\displaystyle 10^{2}+5^{2}+2^{2}}, 8 2 + 8 2 + 1 2 {\displaystyle 8^{2}+8^{2}+1^{2}}, and 8 2 + 7 2 + 4 2 {\displaystyle 8^{2}+7^{2}+4^{2}}. 129 is the product of only two primes, 3 and 43, making 129 a semiprime. Since 3 and 43 are both Gaussian primes, this means that 129 is a Blum integer. 129 is a repdigit in base 6 (333). 129 is a happy number. 129 is a centered octahedral number.