Trivia: date=June 2008

7 (seven) is the natural number following 6 and preceding 8. It is the smallest positive integer to be spoken with two syllables when pronounced in English.

In mathematics

• Seven is the fourth prime number. It is not only a Mersenne prime (since 2³ − 1 = 7) but also a double Mersenne prime since it is itself the exponent for another Mersenne prime (127). It is also a Newman-Shanks-Williams prime, a Woodall prime, a factorial prime, a lucky prime, a happy number and a safe prime.
• Seven is the only number less than 15 which cannot be represented as the sum of the squares of three integers.
• Seven is the aliquot sum of one number, the cubic number 8 and is the base of the 7-aliquot tree.
• n = 7 is the first natural number for which the next statement does not hold: "Two nilpotent endomorphisms from Cⁿ with the same minimal polynomial and the same rank are similar."
• 7 is the only dimension, besides the familiar 3, in which a vector cross product can be definedFact: date=April 2009.
• 999,999 divided by 7 is exactly 142,857. Therefore, when a vulgar fraction with 7 in the denominator is converted to a decimal expansion, the result has the same six-digit repeating sequence after the decimal point, but the sequence can start with any of those six digits. For example, 1/7 = 0.142 857 142.... and 2/7 = 0.285 714 285...
• A seven-sided shape is a heptagon. The regular n-gons for n ≤ 6 can be constructed by compass and straightedge 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.
• There are seven fundamental types of catastrophes.
• When rolling two standard six-sided dice, it is the sum of any two opposite sides on a standard six-sided die. It has a 6 in 36 (or 1/6th) probability of being rolled (1–6, 6–1, 2–5, 5–2, 3–4, or 4–3), the greatest of any number.

In quaternary, 7 is the smallest prime with a composite sum of digits.

In the English language, 7 is the only 2-syllable natural, positive 1-digit number

Evolution of the glyph

In the beginning, various Hindus wrote 7 more or less in one stroke as a curve that looks like an uppercase J vertically inverted. 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, 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. This horizontal stroke is, however, important to distinguish the glyph for seven from the glyph for one in writings that use a long upstroke in the glyph for one. On the seven-segment displays of pocket calculators and digital watches, 7 is the number with the most common glyph variation (0, 6 and 9 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. The reason is that in Japan and Korea 7 is written as ① in the illustration to the right.