991
991 is a prime, odd, a calendar year.
991 (nine hundred ninety-one) is an odd 3-digit number. It is a prime number — divisible only by 1 and itself. Written other ways, in Roman numerals it is CMXCI and in binary, 1111011111.
Interestingness
Historical context — 991 AD
Calendar year
Year 991 (CMXCI) was a common year starting on Thursday of the Julian calendar.
Excerpt from Wikipedia (en) ↗ · Licensed CC BY-SA 4.0 Read the full article on Wikipedia →
Historical context — 991 BC
Decade
The 990s BC is a decade that lasted from 999 BC to 990 BC.
Excerpt from Wikipedia (en) ↗ · Licensed CC BY-SA 4.0 Read the full article on Wikipedia →
Year facts
- Year type
-
Common year
Standard 365-day year; not divisible by 4 (or divisible by 100 but not 400).
- Days in year
- 365
- ISO weeks
- 52
- Started on
-
Saturday
January 1, 991
- Ended on
-
Saturday
December 31, 991
- Friday the 13ths
-
1
One Friday the 13th this year.
- Decade
-
990s
990–999
- Century
-
10th century
901–1000
- Millennium
-
1st millennium
1–1000
- Years ago
-
1,035
1035 years before 2026.
In other calendars
- Hebrew
-
4751 / 4752 AM
Rosh Hashanah falls in September/October.
- Islamic Hijri
-
380 / 381 AH
Lunar calendar; year spans differ from Gregorian.
- Chinese
-
Year of the zodiac:Metal zodiac:Rabbit
Sexagenary cycle position 28 of 60. Lunar new year falls in late January / mid-February.
- Buddhist Era
-
1534 BE
Counted from the parinirvana of the Buddha (Theravada / Thai / Sri Lankan convention).
- Persian Solar Hijri
-
369 / 370 SH
Iranian calendar; Nowruz (new year) falls on the spring equinox.
- Ethiopian
-
983 / 984 ET
Year boundary at Enkutatash (September 11/12).
- Indian National (Saka)
-
913 / 912 Saka
Indian national calendar; year starts in March.
Properties
Primality
991 is prime. It has exactly two divisors: 1 and itself.
Divisors & multiples
Sums & aliquot sequence
Continued fraction of √n
√991 = [31; (2, 12, 10, 2, 2, 2, 1, 1, 2, 6, 1, 1, 1, 1, 3, 1, 8, 4, 1, 2, 1, 2, 3, 1, …)]
Period length 60 — the block in parentheses repeats forever.
Representations
- In words
- nine hundred ninety-one
- Ordinal
- 991st
- Roman numeral
- CMXCI
- Binary
- 1111011111
- Octal
- 1737
- Hexadecimal
- 0x3DF
- Base64
- A98=
- One's complement
- 64,544 (16-bit)
- Scientific notation
- 9.91 × 10²
- As a duration
- 991 s = 16 minutes, 31 seconds
As an angle
Historical numeral systems
- Babylonian (base 60)
- 𒌋𒁹𒁹𒁹𒁹𒁹𒁹 𒌋𒌋𒌋𒁹
- Egyptian hieroglyphic
- 𓍢𓍢𓍢𓍢𓍢𓍢𓍢𓍢𓍢𓎆𓎆𓎆𓎆𓎆𓎆𓎆𓎆𓎆𓏺
- Greek (Milesian)
- ϡϟαʹ
- Mayan (base 20)
- 𝋢·𝋩·𝋫
- Chinese
- 九百九十一
- Chinese (financial)
- 玖佰玖拾壹
Digit at this position in famous constants
- π — Pi (π)
- Digit 991 = 9
- e — Euler's number (e)
- Digit 991 = 8
- φ — Golden ratio (φ)
- Digit 991 = 0
- √2 — Pythagoras's (√2)
- Digit 991 = 2
- ln 2 — Natural log of 2
- Digit 991 = 8
- γ — Euler-Mascheroni (γ)
- Digit 991 = 9
Also seen as
UTF-8 encoding: CF 9F (2 bytes).
As an unsigned 32-bit integer, this is the IPv4 address 0.0.3.223.
- Address
- 0.0.3.223
- Class
- reserved
- IPv4-mapped IPv6
- ::ffff:0.0.3.223
Unspecified address (0.0.0.0/8) — "this network" placeholder.
Heard as a frequency, 991 Hz is closest to:
- Concert pitch (A4 = 440 Hz): B5 (987.8 Hz, +6¢)
- Scientific pitch (C4 = 256 Hz): B5 (966.5 Hz, +43¢)
- Baroque pitch (A4 = 415 Hz): C6 (987 Hz, +7¢)
Related reading
- Prime numbers — The building blocks of arithmetic: what primes are, why they matter, and how we find them.
- Egyptian hieroglyphic numerals — Seven hieroglyphs for every power of ten, from a single stroke to a million.