129 (number)

← 128 129 130 →
← 120 121 122 123 124 125 126 127 128 129 → List of numbers Integers ← 0 100 200 300 400 500 600 700 800 900 →
Cardinal	one hundred twenty-nine
Ordinal	129th (one hundred twenty-ninth)
Factorization	3 × 43
Divisors	1, 3, 43, 129
Greek numeral	ΡΚΘ´
Roman numeral	CXXIX, cxxix
Binary	100000012
Ternary	112103
Senary	3336
Octal	2018
Duodecimal	A912
Hexadecimal	8116

129 (one hundred [and] twenty-nine) is the natural number following 128 and preceding 130.

In mathematics

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: ${\displaystyle 11^{2}+2^{2}+2^{2}}$, ${\displaystyle 10^{2}+5^{2}+2^{2}}$, ${\displaystyle 8^{2}+8^{2}+1^{2}}$, and ${\displaystyle 8^{2}+7^{2}+4^{2}}$.

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. ^[1]

129 is a repdigit in base 6 (333).

129 is a happy number. ^[2]

129 is a centered octahedral number. ^[3]

References

1. "Sloane's A016105 : Blum integers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-27.
2. "Sloane's A007770 : Happy numbers". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation. Retrieved 2016-05-27.
3. Sloane, N. J. A. (ed.). "SequenceA001845(Centered octahedral numbers (crystal ball sequence for cubic lattice))". The On-Line Encyclopedia of Integer Sequences . OEIS Foundation. Retrieved 2022-06-02.