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Number

81

81 is a composite number, odd, a calendar year.

Deficient Number Flippable Harshad / Niven Heptagonal Odious Number Perfect Square Pernicious Number Powerful Number Recamán's Sequence Tribonacci Number Year

Historical context — 81 AD

Calendar year

A.D. 81 (LXXXI) was a common year starting on Monday of the Julian calendar.

Excerpt from Wikipedia (en) ↗ · Licensed CC BY-SA 4.0 · English fallback Read the full article on Wikipedia →

Historical context — 81 BC

Calendar year

Year 81 BC was a year of the pre-Julian Roman calendar.

Excerpt from Wikipedia (en) ↗ · Licensed CC BY-SA 4.0 · English fallback 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: Wednesday
January 1, 81
Ended on: Wednesday
December 31, 81
Friday the 13ths: 1
One Friday the 13th this year.
Decade: 80s
80–89
Century: 1st century
1–100
Millennium: 1st millennium
1–1000
Years ago: 1,945
1945 years before 2026.

In other calendars

Hebrew: 3841 / 3842 AM
Rosh Hashanah falls in September/October.
Chinese: Year of the zodiac:Metal zodiac:Snake
Sexagenary cycle position 18 of 60. Lunar new year falls in late January / mid-February.
Buddhist Era: 624 BE
Counted from the parinirvana of the Buddha (Theravada / Thai / Sri Lankan convention).
Ethiopian: 73 / 74 ET
Year boundary at Enkutatash (September 11/12).
Indian National (Saka): 3 / 2 Saka
Indian national calendar; year starts in March.

Properties

Parity: Odd
Digit count: 2
Digit sum: 9
Digit product: 8
Digital root: 9
Palindrome: No
Bit width: 7 bits
Reversed: 18
Flips to (rotate 180°): 18
Recamán's sequence: a(45) = 81
Square (n²): 6,561
Cube (n³): 531,441
Square root (√n): 9
Divisor count: 5
σ(n) — sum of divisors: 121
φ(n) — Euler's totient: 54
Sum of prime factors: 12

Primality

Prime factorization: 3 ⁴

Nearest primes: 79 (−2) · 83 (+2)

Divisors & multiples

All divisors (5)

1 · 3 · 9 · 27 · 81

Aliquot sum (sum of proper divisors): 40

Factor pairs (a × b = 81)

1 × 81

3 × 27

9 × 9

First multiples

81 · 162 (double) · 243 · 324 · 405 · 486 · 567 · 648 · 729 · 810

Sums & aliquot sequence

As a sum of two squares: 0² + 9²

As consecutive integers: 40 + 41 26 + 27 + 28 11 + 12 + 13 + 14 + 15 + 16 5 + 6 + … + 13

Aliquot sequence: 81 → 40 → 50 → 43 → 1 → 0 — terminates at zero

Representations

In words: eighty-one
Ordinal: 81st
Roman numeral: LXXXI
Binary: 1010001
Octal: 121
Hexadecimal: 0x51
Base64: UQ==
One's complement: 174 (8-bit)

In other bases

ternary (3) 10000

quaternary (4) 1101

quinary (5) 311

senary (6) 213

septenary (7) 144

nonary (9) 100

undecimal (11) 74

duodecimal (12) 69

tridecimal (13) 63

tetradecimal (14) 5b

pentadecimal (15) 56

Historical numeral systems

Babylonian (base 60): 𒁹 𒌋𒌋𒁹
Egyptian hieroglyphic: 𓎆𓎆𓎆𓎆𓎆𓎆𓎆𓎆𓏺
Greek (Milesian): παʹ
Mayan (base 20): 𝋤·𝋡
Chinese: 八十一
Chinese (financial): 捌拾壹

In other modern scripts

Eastern Arabic ٨١ Devanagari ८१ Bengali ৮১ Tamil ௮௧ Thai ๘๑ Tibetan ༨༡ Khmer ៨១ Lao ໘໑ Burmese ၈၁

Digit at this position in famous constants

π — Pi (π): Digit 81 = 9
e — Euler's number (e): Digit 81 = 9
φ — Golden ratio (φ): Digit 81 = 4
√2 — Pythagoras's (√2): Digit 81 = 3
ln 2 — Natural log of 2: Digit 81 = 1
γ — Euler-Mascheroni (γ): Digit 81 = 7

Also seen as

ASCII character

As an ASCII codepoint, 81 is Q. Printable ASCII character Q.

Hex color

#000051

RGB(0, 0, 81)

IPv4 address

As an unsigned 32-bit integer, this is the IPv4 address 0.0.0.81.

Address: 0.0.0.81
Class: reserved
IPv4-mapped IPv6: ::ffff:0.0.0.81

Unspecified address (0.0.0.0/8) — "this network" placeholder.

US numbered highway

Matches numbered highway designation:

I-81 — Dandridge, TN to Wellesley Island, NY (Canadian border).

Interstate 81 on Wikipedia ↗