28
28 is a composite number, even, a calendar year.
28 (twenty-eight) is an even 2-digit number. It is a composite number with 6 divisors, and factors as 2² × 7. Remarkably, it is a perfect number: the sum of its proper divisors is exactly 28, equal to the number itself — a property shared by only a handful of known integers. It is the seventh triangular number. Written other ways, in Roman numerals it is XXVIII and in binary, 11100.
Interestingness
Historical context — 28 AD
Calendar year
AD 28 (XXVIII) was a leap year starting on Thursday of the Julian calendar.
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Historical context — 28 BC
Calendar year
Year 28 BC was either a common year starting on Saturday, Sunday or Monday or a leap year starting on Saturday or Sunday of the Julian calendar and a common year starting on Saturday of the Proleptic Julian calendar.
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Year facts
- Year type
-
Leap year
Divisible by 4 and not by 100; February has 29 days.
- Days in year
- 366
- ISO weeks
- 52
- Started on
-
Saturday
January 1, 28
- Ended on
-
Sunday
December 31, 28
- Friday the 13ths
-
1
One Friday the 13th this year.
- Decade
-
20s
20–29
- Century
-
1st century
1–100
- Millennium
-
1st millennium
1–1000
- Years ago
-
1,998
1998 years before 2026.
In other calendars
- Hebrew
-
3788 / 3789 AM
Rosh Hashanah falls in September/October.
- Chinese
-
Year of the zodiac:Earth zodiac:Rat
Sexagenary cycle position 25 of 60. Lunar new year falls in late January / mid-February.
- Buddhist Era
-
571 BE
Counted from the parinirvana of the Buddha (Theravada / Thai / Sri Lankan convention).
- Ethiopian
-
20 / 21 ET
Year boundary at Enkutatash (September 11/12).
- Indian National (Saka)
-
-50 / -51 Saka
Indian national calendar; year starts in March.
Properties
Primality
Prime factorization: 2 2 × 7
Divisors & multiples
Sums & aliquot sequence
Continued fraction of √n
√28 = [5; (3, 2, 3, 10)]
Period length 4 — the block in parentheses repeats forever.
Representations
- In words
- twenty-eight
- Ordinal
- 28th
- Roman numeral
- XXVIII
- Binary
- 11100
- Octal
- 34
- Hexadecimal
- 0x1C
- Base64
- HA==
- One's complement
- 227 (8-bit)
- Scientific notation
- 2.8 × 10¹
As an angle
Historical numeral systems
- Babylonian (base 60)
- 𒌋𒌋𒁹𒁹𒁹𒁹𒁹𒁹𒁹𒁹
- Egyptian hieroglyphic
- 𓎆𓎆𓏺𓏺𓏺𓏺𓏺𓏺𓏺𓏺
- Greek (Milesian)
- κηʹ
- Mayan (base 20)
- 𝋡·𝋨
- Chinese
- 二十八
- Chinese (financial)
- 貳拾捌
Digit at this position in famous constants
- π — Pi (π)
- Digit 28 = 3
- e — Euler's number (e)
- Digit 28 = 1
- φ — Golden ratio (φ)
- Digit 28 = 4
- √2 — Pythagoras's (√2)
- Digit 28 = 4
- ln 2 — Natural log of 2
- Digit 28 = 1
- γ — Euler-Mascheroni (γ)
- Digit 28 = 0
Also seen as
Goldbach's conjecture says every even integer greater than 2 is the sum of two primes. For 28, here are decompositions:
- 5 + 23 = 28
- 11 + 17 = 28
As an ASCII codepoint, 28 is control character (0x1C). ASCII control character.
As an unsigned 32-bit integer, this is the IPv4 address 0.0.0.28.
- Address
- 0.0.0.28
- Class
- reserved
- IPv4-mapped IPv6
- ::ffff:0.0.0.28
Unspecified address (0.0.0.0/8) — "this network" placeholder.
Heard as a frequency, 28 Hz is closest to:
- Concert pitch (A4 = 440 Hz): A0 (27.5 Hz, +31¢)
- Scientific pitch (C4 = 256 Hz): A♯0 (28.5 Hz, -31¢)
- Baroque pitch (A4 = 415 Hz): A♯0 (27.5 Hz, +32¢)
As a MIDI note number, 28 is E1 (41.2 Hz at concert pitch). (very low — many laptop and phone speakers can't reproduce it)
On the periodic table, atomic number 28 is Nickel (Ni) — period 4.
Related reading
- Perfect numbers — Numbers exactly equal to the sum of their parts — only 52 are known, and the odd ones may not exist at all.
- Triangular numbers — 1, 3, 6, 10, 15 … the counting numbers stacked into triangles, and Gauss's famous shortcut for summing them.
- Egyptian hieroglyphic numerals — Seven hieroglyphs for every power of ten, from a single stroke to a million.