It is a composite number; a square-prime, of the form (p2,q) where q is a higher prime. It is the third of this form and of the specific form (22.q), with proper divisors being 1, 2, 4, 7, and 14.
Twenty-eight is the second perfect number - it is the sum of its proper divisors: . As a perfect number, it is related to the Mersenne prime 7, since . The next perfect number is 496, the previous being 6.[1]
Though perfect, 28 is not the aliquot sum of any other number other than itself, and so; unusually, is not part of a multi-number aliquot sequence.
The next perfect number is 496.
Twenty-eight is the sum of the totient function for the first nine integers.[2]
Twenty-eight is the only positive integer that has a unique Kaylesnim-value.
Twenty-eight is the only known number that can be expressed as a sum of the first nonnegative (or positive) integers (), a sum of the first primes () and a sum of the first nonprimes (), and it is unlikely that any other number has this property.[11]
The number of players on the active roster of teams in Nippon Professional Baseball. However, each team is limited to using 25 players in a given game; before every game, the manager must designate three players who will be ineligible for that game.
From 2020, the number of players on the active roster of Major League Baseball teams for regular-season games on or after September 1.
The number of days in the shortest month of the Gregorian calendar, February in common years. All twelve months of the Gregorian calendar have at least 28 days, regardless of the year.
The Gregorian calendar follows a 28-year cycle for the most part, since there are seven days in a week and leap year generally occurs every four years; usually, a calendar from any year is the same as that from 28 years earlier (e.g., 1992 and 2020 or 1994 and 2022). However, that rule holds only when there have been exactly seven leap days in a 28-year interval; years divisible by 100 but not by 400 are common years. Indeed, 1900 (as well as 2100, 2200, etc.) does not use the same calendar as 1872 (2072, 2172, etc., respectively) for the simple reason that 1900 is a common year. In 28 years, any day-of-the-week and date combination occurs exactly four times. February 29 will fall on each day of the week once.
In Jewish tradition there is a 28-year solar cycle in which the sun returns to its place in Creation every 28 solar years. This is commemorated in April every 28 years with the recitation of Birkat Hachama, the blessing of the sun.
Deriving from the 29.46 year period of Saturn's revolution around the Sun, the 28-year cycle as well as its subdivisions by 14 and 7 are supposed in astrology to mark significant turning points or sections in the course of a person's development in life. Thus, the number 28 has special significance in the culture of religious sects such as the Kadiri and the Mevlevi dervishes. The 28-beat metric pattern often used in the music compositions accompanying the main part of the Mevlevi sema ritual is called the "Devri kebir", meaning the "Big Circle" and is a reference to above astronomical facts about the year and the Saturn year.
In Quebec, François Pérusse, in one of his best-selling Album du peuple made a parody of Wheel of Fortune in which all of the letters picked by the contestant were present 28 times. As a result, 28 became an almost mythical number used by many Quebec youths, the phrase "Y'en a 28" (There are 28 [Letters]) became a running gag still used and recognized more than 15 years later.
The Preludes, Opus 28 consists of Frédéric Chopin's 24 preludes for piano, ordinarily but not necessarily played together in concert.
The number of Panfilov's Guardsmen, said to have heroically fallen in combat on 16 November 1941 during the Battle of Moscow, and venerated as Soviet's national heroes.
In Cockney Rhyming slang, two and eight means (mental) state, typically a poor state. Example: He was in a right two and eight. [18]