Number reference
131 explained.
131: one hundred thirty-one, prime, palindromic prime; factors, prime factorization, roots, binary, hexadecimal, and digit properties.
Identity and representations
- Written form
- one hundred thirty-one
- Classification
- prime
- Factors
- 1, 131
- Prime factorization
- 131
- Square root
- 11.445523
- Cube root
- 5.078753
- Binary
- 10000011
- Octal
- 203
- Hexadecimal
- 83
- Roman numeral
- CXXXI
- Digit sum
- 5
- Digital root
- 5
What makes 131 notable
- Prime. 131 is prime: it has exactly two divisors, 1 and itself, so it cannot be split into equal whole-number groups any other way.
- Palindromic prime. 131 is prime and reads the same forwards and backwards.
131 in everyday use
One hundred thirty-one is best known through iodine-131, a radioactive isotope with a half-life of about eight days. It is used in medicine to treat overactive thyroids and thyroid cancer, because the thyroid absorbs iodine and the isotope delivers its radiation exactly there. The same property makes it a health concern after nuclear accidents, which is why potassium iodide tablets are stockpiled near reactors: filling the thyroid with ordinary iodine blocks the radioactive kind. The Chernobyl and Fukushima releases both put iodine-131 in the news.
As a number, 131 is a palindromic prime, and it belongs to the permutable-prime trio with 113 and 311, every rearrangement of its digits being prime. It is also a Sophie Germain prime, since 2 × 131 + 1 = 263 is prime. Its position among primes is the 32nd, and its digit sum of 5 is one of the few things about it that is not itself prime — 5 is prime too.
Nearby numbers
How to read this page
Because 131 is prime, the factor list stops at 1 and 131 itself, so it cannot be shared into equal whole-number groups. That is also what makes it useful as a building block: every larger number with 131 in its prime factorisation inherits it from here.
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