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7 (number)

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Cardinal 7
Ordinal 7th
Numeral system septenary
Factorization prime
Divisors 1, 7
Roman numeral VII
Unicode representation of Roman numeral Ⅶ, ⅶ
prefixes hepta-/hept- (from Greek)

septua- (from Latin)

Binary 111
Octal 7
Duodecimal 7
Hexadecimal 7
Hebrew ז (Zayin)

7 (seven) is the natural number following 6 and preceding 8.

The SI prefix for 10007 is zetta (Z), and for its reciprocal zepto (z).


Evolution of the glyph

In the beginning, various Indians wrote 7 more or less in one stroke as a curve that looks like an uppercase J upside down. The western Ghubar Arabs' main contribution was to make the longer line diagonal rather than straight, though they showed some tendencies to making the character more rectilinear. The eastern Arabs developed the character from a 6-look-alike into an uppercase V-look-alike. Both modern Arab forms influenced the European form, a two-stroke character consisting of a horizontal upper line joined at its right to a line going down to the bottom left corner, a line that is slightly curved in some font variants. As is the case with the European glyph, the Cham and Khmer glyph for 7 also evolved to look like their glyph for 1, though in a different way, and so they were also concerned with making their 7 more different. For the Khmer this often involved adding a horizontal line above the glyph. This is analogous to the horizontal stroke through the middle that is sometimes used in handwriting in the Western world but which is almost never used in computer fonts.

On the seven-segment displays of pocket calculators and digital watches, 7 is the number with the most common glyph variation (0 and 6 also have variant glyphs). Most calculators use three line segments, but on Sharp, Casio and a few other brands of calculators, 7 is written with four line segments.

In mathematics

Seven is the fourth-smallest prime number; the next is 11. 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

Base Numeral system
2 binary 111
3 ternary 21
4 quaternary 13
5 quinary 12
6 senary 11
7 septenary 10
over 7 (octal, decimal) 7

In science

In astronomy,

The number of objects in the solar system visible to the naked eye (Moon, Mars, Mercury, Jupiter, Venus, Saturn, Sun). A fact that is believed to have led to the concept of seven days of the week first adopted by the Babylonians.
Messier object M7, a magnitude 3.5 open cluster in the constellation Scorpius.
The New General Catalogue object NGC7, a 14th magnitude spiral galaxy in the constellation Sculptor
The Saros number of the solar eclipse series which began on -2590 April 8 and ended on -1310 May 16. The duration of Saros series 7 was 1280.1 years, and it contained 72 solar eclipses.
The Saros number of the lunar eclipse series which began on -2595 July 15 and ended on -1008 February 22. The duration of Saros series 7 was 1586.6 years, and it contained 89 lunar eclipses.

In other fields

Seven is the smallest positive integer requiring two syllables in English.

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 diameter.

Many cities are claimed to be constructed amidst seven hills; see Seven hills of Rome and List of cities claiming to be built on seven hills.

Seven is also:

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