7 (number)

7 (seven) is the natural number following 6 and preceding 8. Seven is the smallest positive integer requiring two syllables in English.

In mathematics

Seven is the fourth-smallest prime number; the next is eleven. Seven is a Mersenne prime, since 2³ - 1 = 7. Seven is also the fourth Mersenne prime exponent, a Newman-Shanks-Williams prime, and a Woodall prime. 7 is also the second lucky prime.

999,999 divided by 7 is exactly 142,857, so fractions with 7 in the denominator have six-digit repeating sequences in their decimal expansions. 1/7 = 0.142857142....

A seven-sided polygon is a heptagon. The regular n-gons for n ≤ 6 can be constructed by ruler and compass alone, but the regular heptagon cannot. Figurate numbers representing heptagons (including seven) are called heptagonal numbers. Seven is also a centered hexagonal number.

There are seven frieze groups, the groups consisting of symmetries of the plane whose group of translations is isomorphic to the group of integers.

In numeral systems

In other fields

There are seven musicians in a septet and seven babies born in a set of septuplets. The most famous set of septuplets are the McCaughey Septuplets, who were born in 1997.

The British fifty-pence and twenty-pence coins are heptagons, with the sides curved to give them a constant radius.

Cities constructed amidst seven hills include:

• Rome, see Seven hills of Rome
• Cincinnati
• Pretoria
• Lynchburg, Virginia
• Bath, England

Seven is also:

• The atomic number of nitrogen.
• The number of periods, or horizontal rows of elements, on the periodic table.
• The number of carbon atoms in Heptane, a hydrocarbon.
• the traditional number of Wonders of the Ancient World.
• viewed as a lucky number in many Western cultures, and in Japanese culture. In Western cultures it is considered lucky because the numbers three and four added together equal 7. Those two numbers are lucky as well, with 7 being extra lucky.
• the number of days in a week.
• a highly symbolic number in Christian thought (being the number of days in which God created the earth, etc)
• referred to in bingo as 'Lucky Seven'.
• the number of the Deadly Sins: lust, avarice, envy, pride, sloth, gluttony and anger.
• considered the usual number for a witches' coven.
• the neutral pH level.
• the number of palms in an Egyptian Sacred Cubit.
• the number of the ages of man into which William Shakespeare divided a lifetime.
• the figurative number of seas.
• the number of colors of the rainbow.
• the number of basic principles of the bushido.
• the number of heads of the beast of Revelations, and of some other monsters, like the hydra.
• the number of points on a sheriff's star.
• the code for international direct dial phone calls to Russia and Kazakhstan.
• the title of:
  • a 2003 album by Enrique Iglesias, see 7 (Enrique Iglesias album).
  • the film Seven (or Se7en), directed in 1995 by David Fincher and starring Morgan Freeman, Brad Pitt and Kevin Spacey.
• the name of one of the monsters in the cartoon television show Seven Little Monsters.
• the name George Costanza desired to give his first-born (with Susan Ross due to a promise he had made to the widow of Micky Mantle, whose number was 7.
• the number of digits in an American telephone number, excluding the area code.
• the number of "Zoomers" on the PBS children's show Zoom.
• the number of sacraments in the Roman Catholic faith.
• the number of individual segments in one digit on a digital clock or calculator screen.
• part of the names of:
  • Seven Pillars of Wisdom, a book written by T. E. Lawrence.
  • the days in the movie The Ring until you die. "You will die in 7 days."
  • the film Seven Years in Tibet
  • Several groupings called the Seven Sisters.
  • Seven Sisters, a 1998 song album by pop singer Meja.
  • the crewmember, Seven of Nine, in the Star Trek universe.
  • the World Sevens Series in Rugby Union, including the Hong Kong Sevens.
  • The Seven Habits of Highly Successful People, a book by Stephen Covey.
• In rugby union, the openside flanker
• Historical years: 7 A.D., 7 B.C., or 1907