Number reference
91 explained.
91: ninety-one, composite, triangular number; factors, prime factorization, roots, binary, hexadecimal, and digit properties.
Identity and representations
- Written form
- ninety-one
- Classification
- composite
- Factors
- 1, 7, 13, 91
- Prime factorization
- 7 × 13
- Square root
- 9.539392
- Cube root
- 4.497941
- Binary
- 1011011
- Octal
- 133
- Hexadecimal
- 5B
- Roman numeral
- XCI
- Digit sum
- 10
- Digital root
- 1
What makes 91 notable
- Triangular number. 91 is the 13th triangular number: it is the sum 1 + 2 + … + 13, the count of items in a triangle with 13 rows.
91 in everyday use
Ninety-one days is a quarter of a year to the nearest day — thirteen weeks — which is why quarterly bills and reports often use it. Ninety-one is the atomic number of protactinium, a rare radioactive metal with no practical uses outside research. In the United States, Interstate 91 runs from New Haven, Connecticut, to the Canadian border in Vermont. The 1991 dissolution of the Soviet Union ended the Cold War.
Ninety-one is 7 × 13, a classic false prime. It is the thirteenth triangular number, 1 + 2 + … + 13, and a square pyramidal number, 1 + 4 + 9 + 16 + 25 + 36. It is the sum of two cubes, 27 + 64, and also the difference of two cubes, 216 − 125. That makes it one of very few numbers that are both triangular and square-pyramidal, alongside 1 and 55.
Nearby numbers
How to read this page
The factor list shows every way 91 splits into equal whole-number groups — 4 divisors in total. The prime factorisation is the compact form of that same information, and the base conversions restate 91 for contexts that do not use decimal.
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