Number

20

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

20 (twenty) is an even 2-digit number. It is a composite number with 6 divisors, and factors as 2² × 5. Its proper divisors sum to 22, more than the number itself, making it an abundant number. Written other ways, in Roman numerals it is XX and in binary, 10100.

Abundant Number Arithmetic Number Cube-Free Evil Number Harshad / Niven Highly Abundant Pernicious Number Practical Number Preferred Number Pronic / Oblong Recamán's Sequence Self Number Semiperfect Number Tetrahedral Year

Interestingness

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11 notable facts · grade A

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Historical context — 20 AD

Calendar year

AD 20 (XX) was a leap year starting on Monday of the Julian calendar.

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

Historical context — 20 BC

Calendar year

Year 20 BC was either a common year starting on Wednesday or Thursday or a leap year starting on Tuesday, Wednesday or Thursday of the Julian calendar and a common year starting on Tuesday of the Proleptic Julian calendar.

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

Year facts

Year type: Leap year
Divisible by 4 and not by 100; February has 29 days.
Days in year: 366
ISO weeks: 53
Long year: contains 53 ISO weeks.
Started on: Wednesday
January 1, 20
Ended on: Thursday
December 31, 20
Friday the 13ths: 2
2 Friday the 13ths this year.
Decade: 20s
20–29
Century: 1st century
1–100
Millennium: 1st millennium
1–1000
Years ago: 2,006
2006 years before 2026.

In other calendars

Hebrew: 3780 / 3781 AM
Rosh Hashanah falls in September/October.
Chinese: Year of the zodiac:Metal zodiac:Dragon
Sexagenary cycle position 17 of 60. Lunar new year falls in late January / mid-February.
Buddhist Era: 563 BE
Counted from the parinirvana of the Buddha (Theravada / Thai / Sri Lankan convention).
Ethiopian: 12 / 13 ET
Year boundary at Enkutatash (September 11/12).
Indian National (Saka): -58 / -59 Saka
Indian national calendar; year starts in March.

Properties

Parity: Even
Digit count: 2
Digit sum: 2
Digit product: 0
Digital root: 2
Palindrome: No
Bit width: 5 bits
Reversed: 2
Recamán's sequence: a(7) = 20
Square (n²): 400
Cube (n³): 8,000
Divisor count: 6
σ(n) — sum of divisors: 42
φ(n) — Euler's totient: 8
Sum of prime factors: 9

Primality

Prime factorization: 2 ² × 5

Nearest primes: 19 (−1) · 23 (+3)

Divisors & multiples

All divisors (6)

1 · 2 · 4 · 5 · 10 (half) · 20

Aliquot sum (sum of proper divisors): 22

Factor pairs (a × b = 20)

1 × 20

2 × 10

4 × 5

First multiples

20 · 40 (double) · 60 · 80 · 100 · 120 · 140 · 160 · 180 · 200

Sums & aliquot sequence

As a sum of two squares: 2² + 4²

As consecutive integers: 2 + 3 + 4 + 5 + 6

Aliquot sequence: 20 → 22 → 14 → 10 → 8 → 7 → 1 → 0 — terminates at zero

Continued fraction of √n

√20 = [4; (2, 8)]

Period length 2 — the block in parentheses repeats forever.

Representations

In words: twenty
Ordinal: 20th
Roman numeral: XX
Binary: 10100
Octal: 24
Hexadecimal: 0x14
Base64: FA==
One's complement: 235 (8-bit)
Scientific notation: 2 × 10¹

In other bases

ternary (3) 202

quaternary (4) 110

quinary (5) 40

senary (6) 32

septenary (7) 26

nonary (9) 22

undecimal (11) 19

duodecimal (12) 18

tridecimal (13) 17

tetradecimal (14) 16

pentadecimal (15) 15

Palindromic in base 3

As an angle

20°

20° ≈ 0.349 rad

Compass bearing: NNE (north-northeast)

Why does a circle have 360 degrees? →

Historical numeral systems

Babylonian (base 60): 𒌋𒌋
Egyptian hieroglyphic: 𓎆𓎆
Greek (Milesian): κʹ
Mayan (base 20): 𝋡·𝋠
Chinese: 二十
Chinese (financial): 貳拾

In other modern scripts

Eastern Arabic ٢٠ Devanagari २० Bengali ২০ Tamil ௨௦ Thai ๒๐ Tibetan ༢༠ Khmer ២០ Lao ໒໐ Burmese ၂၀

Digit at this position in famous constants

π — Pi (π): Digit 20 = 4
e — Euler's number (e): Digit 20 = 3
φ — Golden ratio (φ): Digit 20 = 2
√2 — Pythagoras's (√2): Digit 20 = 8
ln 2 — Natural log of 2: Digit 20 = 4
γ — Euler-Mascheroni (γ): Digit 20 = 6

Also seen as

Goldbach decomposition

Goldbach's conjecture says every even integer greater than 2 is the sum of two primes. For 20, here are decompositions:

3 + 17 = 20
7 + 13 = 20

ASCII character

As an ASCII codepoint, 20 is control character (0x14). ASCII control character.

Network port

TCP/UDP port 20 is the well-known port for FTP-DATA — File Transfer Protocol — data channel.

Hex color

#000014

RGB(0, 0, 20)

IPv4 address

As an unsigned 32-bit integer, this is the IPv4 address 0.0.0.20.

Address: 0.0.0.20
Class: reserved
IPv4-mapped IPv6: ::ffff:0.0.0.20

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

US numbered highway

Matches numbered highway designations:

I-20 — Kent, TX to Florence, SC.
US 20 — Boston, MA to Newport, OR — the longest US Route (3,365 mi).

Interstate 20 on Wikipedia ↗ US 20 on Wikipedia ↗

Musical pitch

Heard as a frequency, 20 Hz is closest to:

Concert pitch (A4 = 440 Hz): D♯0 (19.4 Hz, +49¢ — about midway to E0)
Scientific pitch (C4 = 256 Hz): E0 (20.2 Hz, -14¢)
Baroque pitch (A4 = 415 Hz): E0 (19.4 Hz, +50¢ — about midway to F0)

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

As a MIDI note number, 20 is G♯0 (26 Hz at concert pitch). (very low — many laptop and phone speakers can't reproduce it)

Atomic number

On the periodic table, atomic number 20 is Calcium (Ca) — period 4.

Calcium on Wikipedia ↗