Number

991

991 is a prime, odd, a calendar year.

Arithmetic Number Chen Prime Deficient Number Emirp Flippable Odious Number Prime Recamán's Sequence Sexy Prime Squarefree Year

Historical context — 991 AD

Calendar year

Year 991 (CMXCI) was a common year starting on Thursday of the Julian calendar.

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Historical context — 991 BC

Decade

The 990s BC is a decade that lasted from 999 BC to 990 BC.

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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

Parity: Odd
Digit count: 3
Digit sum: 19
Digit product: 81
Digital root: 1
Palindrome: No
Bit width: 10 bits
Reversed: 199
Flips to (rotate 180°): 166
Recamán's sequence: a(4,437) = 991
Square (n²): 982,081
Cube (n³): 973,242,271
Divisor count: 2
σ(n) — sum of divisors: 992
φ(n) — Euler's totient: 990

Primality

991 is prime. It has exactly two divisors: 1 and itself.

Divisors & multiples

All divisors (2)

1 · 991

Aliquot sum (sum of proper divisors): 1

Factor pairs (a × b = 991)

1 × 991

First multiples

991 · 1,982 (double) · 2,973 · 3,964 · 4,955 · 5,946 · 6,937 · 7,928 · 8,919 · 9,910

Sums & aliquot sequence

As consecutive integers: 495 + 496

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)

In other bases

ternary (3) 1100201

quaternary (4) 33133

quinary (5) 12431

senary (6) 4331

septenary (7) 2614

nonary (9) 1321

undecimal (11) 821

duodecimal (12) 6a7

tridecimal (13) 5b3

tetradecimal (14) 50b

pentadecimal (15) 461

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

In other modern scripts

Eastern Arabic ٩٩١ Devanagari ९९१ Bengali ৯৯১ Tamil ௯௯௧ Thai ๙๙๑ Tibetan ༩༩༡ Khmer ៩៩១ Lao ໙໙໑ Burmese ၉၉၁

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

Prime neighborhood

Adjacent primes:

Previous prime: 983 (gap of 8)
Next prime: 997 (gap of 6)

Pair status: sexy with 997.

Unicode codepoint

Greek Small Letter Koppa

U+03DF

Lowercase letter (Ll)

UTF-8 encoding: CF 9F (2 bytes).

Lookup U+03DF on Compart ↗ Lookup on FileFormat.Info ↗

Hex color

#0003DF

RGB(0, 3, 223)

IPv4 address

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.