Number

66

66 is a composite number, even, a calendar year.

66 (sixty-six) is an even 2-digit number. It is a composite number with 8 divisors, and factors as 2 × 3 × 11. Its proper divisors sum to 78, more than the number itself, making it an abundant number. It is the eleventh triangular number. Its digits read the same forwards and backwards, so it is a palindromic number. Written other ways, in Roman numerals it is LXVI and in binary, 1000010.

Abundant Number Arithmetic Number Cube-Free Evil Number Flippable Hexagonal Palindrome Pernicious Number Practical Number Recamán's Sequence Repdigit Semiperfect Number Sphenic Number Squarefree Triangular Year

Interestingness

★ ★ ★ ★ ★

12 notable facts · grade A

54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78

less more

Historical context — 66 AD

Calendar year

AD 66 (LXVI) was a common year starting on Wednesday of the Julian calendar.

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

Historical context — 66 BC

Calendar year

Year 66 BC was a year of the pre-Julian Roman calendar.

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: Friday
January 1, 66
Ended on: Friday
December 31, 66
Friday the 13ths: 1
One Friday the 13th this year.
Decade: 60s
60–69
Century: 1st century
1–100
Millennium: 1st millennium
1–1000
Years ago: 1,960
1960 years before 2026.

In other calendars

Hebrew: 3826 / 3827 AM
Rosh Hashanah falls in September/October.
Chinese: Year of the zodiac:Fire zodiac:Tiger
Sexagenary cycle position 3 of 60. Lunar new year falls in late January / mid-February.
Buddhist Era: 609 BE
Counted from the parinirvana of the Buddha (Theravada / Thai / Sri Lankan convention).
Ethiopian: 58 / 59 ET
Year boundary at Enkutatash (September 11/12).
Indian National (Saka): -12 / -13 Saka
Indian national calendar; year starts in March.

Properties

Parity: Even
Digit count: 2
Digit sum: 12
Digit product: 36
Digital root: 3
Palindrome: Yes
Bit width: 7 bits
Flips to (rotate 180°): 99
Recamán's sequence: a(95) = 66
Square (n²): 4,356
Cube (n³): 287,496
Divisor count: 8
σ(n) — sum of divisors: 144
φ(n) — Euler's totient: 20
Sum of prime factors: 16

Primality

Prime factorization: 2 × 3 × 11

Nearest primes: 61 (−5) · 67 (+1)

Divisors & multiples

All divisors (8)

1 · 2 · 3 · 6 · 11 · 22 · 33 (half) · 66

Aliquot sum (sum of proper divisors): 78

Factor pairs (a × b = 66)

1 × 66

2 × 33

3 × 22

6 × 11

First multiples

66 · 132 (double) · 198 · 264 · 330 · 396 · 462 · 528 · 594 · 660

Sums & aliquot sequence

As consecutive integers: 21 + 22 + 23 15 + 16 + 17 + 18 1 + 2 + … + 11

Aliquot sequence: 66 → 78 → 90 → 144 → 259 → 45 → 33 → 15 → 9 → 4 → 3 → 1 → 0 — terminates at zero

Continued fraction of √n

√66 = [8; (8, 16)]

Period length 2 — the block in parentheses repeats forever.

Representations

In words: sixty-six
Ordinal: 66th
Roman numeral: LXVI
Binary: 1000010
Octal: 102
Hexadecimal: 0x42
Base64: Qg==
One's complement: 189 (8-bit)
Scientific notation: 6.6 × 10¹
As a duration: 66 s = 1 minute, 6 seconds

In other bases

ternary (3) 2110

quaternary (4) 1002

quinary (5) 231

senary (6) 150

septenary (7) 123

nonary (9) 73

undecimal (11) 60

duodecimal (12) 56

tridecimal (13) 51

tetradecimal (14) 4a

pentadecimal (15) 46

As an angle

66°

66° ≈ 1.152 rad

Compass bearing: ENE (east-northeast)

Why does a circle have 360 degrees? →

Historical numeral systems

Babylonian (base 60): 𒁹 𒁹𒁹𒁹𒁹𒁹𒁹
Egyptian hieroglyphic: 𓎆𓎆𓎆𓎆𓎆𓎆𓏺𓏺𓏺𓏺𓏺𓏺
Greek (Milesian): ξϛʹ
Mayan (base 20): 𝋣·𝋦
Chinese: 六十六
Chinese (financial): 陸拾陸

In other modern scripts

Eastern Arabic ٦٦ Devanagari ६६ Bengali ৬৬ Tamil ௬௬ Thai ๖๖ Tibetan ༦༦ Khmer ៦៦ Lao ໖໖ Burmese ၆၆

Digit at this position in famous constants

π — Pi (π): Digit 66 = 0
e — Euler's number (e): Digit 66 = 2
φ — Golden ratio (φ): Digit 66 = 0
√2 — Pythagoras's (√2): Digit 66 = 9
ln 2 — Natural log of 2: Digit 66 = 9
γ — Euler-Mascheroni (γ): Digit 66 = 8

Also seen as

Goldbach decomposition

Goldbach's conjecture says every even integer greater than 2 is the sum of two primes. For 66, here are decompositions:

5 + 61 = 66
7 + 59 = 66
13 + 53 = 66
19 + 47 = 66
23 + 43 = 66
29 + 37 = 66

ASCII character

As an ASCII codepoint, 66 is B. Printable ASCII character B.

Hex color

#000042

RGB(0, 0, 66)

IPv4 address

As an unsigned 32-bit integer, this is the IPv4 address 0.0.0.66.

Address: 0.0.0.66
Class: reserved
IPv4-mapped IPv6: ::ffff:0.0.0.66

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

US numbered highway

Matches numbered highway designations:

I-66 — Strasburg, VA to Washington, D.C.
US 66 — (decommissioned 1985) Chicago to Santa Monica — "the Mother Road" of American culture and song.

Interstate 66 on Wikipedia ↗ US 66 on Wikipedia ↗

Geographic coordinate

As a geographic coordinate in degrees, this matches:

Arctic / Antarctic Circle (latitude) — Approximately 66.5° latitude — the latitude beyond which the Sun does not set on the summer solstice (or rise on the winter solstice).

World map (equirectangular projection) — Latitude 66° — Arctic / Antarctic Circle

Musical pitch

Heard as a frequency, 66 Hz is closest to:

Concert pitch (A4 = 440 Hz): C2 (65.4 Hz, +16¢)
Scientific pitch (C4 = 256 Hz): C♯2 (67.8 Hz, -47¢ — about midway to C2)
Baroque pitch (A4 = 415 Hz): C♯2 (65.4 Hz, +17¢)

(very low — many laptop and phone speakers can't reproduce it)

As a MIDI note number, 66 is F♯4 (370 Hz at concert pitch).

Atomic number

On the periodic table, atomic number 66 is Dysprosium (Dy) — period 6.

Dysprosium on Wikipedia ↗