For early Brahmi numerals, 7 was written more or less in one stroke as a curve that looks like an uppercase ⟨J⟩ vertically inverted (ᒉ). The western Arab peoples' main contribution was to make the longer line diagonal rather than straight, though they showed some tendencies to making the digit more rectilinear. The eastern Arab peoples developed the digit from a form that looked something like 6 to one that looked like an uppercase V. Both modern Arab forms influenced the European form, a two-stroke form consisting of a horizontal upper stroke joined at its right to a stroke going down to the bottom left corner, a line that is slightly curved in some font variants. As is the case with the European digit, the Cham and Khmer digit for 7 also evolved to look like their digit 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 to the top of the digit.[2] 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 writing that uses a long upstroke in the glyph for 1. In some Greek dialects of the early 12thcentury the longer line diagonal was drawn in a rather semicircular transverse line.
On seven-segment displays, 7 is the digit with the most common graphic variation (1, 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 because in Japan, Korea and Taiwan 7 is written with a "hook" on the left, as ① in the following illustration.
While the shape of the character for the digit 7 has an ascender in most modern typefaces, in typefaces with text figures the character usually has a descender (⁊), as, for example, in .
Most people in Continental Europe,[3] Indonesia,[citation needed] and some in Britain, Ireland, and Canada, as well as Latin America, write 7 with a line through the middle (7), sometimes with the top line crooked. The line through the middle is useful to clearly differentiate the digit from the digit one, as they can appear similar when written in certain styles of handwriting. This form is used in official handwriting rules for primary school in Russia, Ukraine, Bulgaria, Poland, other Slavic countries,[4] France,[5] Italy, Belgium, the Netherlands, Finland,[6] Romania, Germany, Greece,[7] and Hungary.[citation needed]
A seven-sided shape is a heptagon.[15] The regularn-gons for n ⩽ 6 can be constructed by compass and straightedge alone, which makes the heptagon the first regular polygon that cannot be directly constructed with these simple tools.[16]
7 is the only number D for which the equation 2n − D = x2 has more than two solutions for n and xnatural. In particular, the equation 2n − 7 = x2 is known as the Ramanujan–Nagell equation. 7 is one of seven numbers in the positive definite quadraticinteger matrix representative of all odd numbers: {1, 3, 5, 7, 11, 15, 33}.[17][18]
A heptagon in Euclidean space is unable to generate uniform tilings alongside other polygons, like the regular pentagon. However, it is one of fourteen polygons that can fill a plane-vertex tiling, in its case only alongside a regular triangle and a 42-sided polygon (3.7.42).[22][23] This is also one of twenty-one such configurations from seventeen combinations of polygons, that features the largest and smallest polygons possible.[24][25] Otherwise, for any regular n-sided polygon, the maximum number of intersecting diagonals (other than through its center) is at most 7.[26]
In two dimensions, there are precisely seven 7-uniformKrotenheerdt tilings, with no other such k-uniform tilings for k > 7, and it is also the only k for which the count of Krotenheerdt tilings agrees with k.[27][28]
In hyperbolic space, 7 is the highest dimension for non-simplex hypercompact Vinberg polytopes of rank n + 4 mirrors, where there is one unique figure with eleven facets. On the other hand, such figures with rank n + 3 mirrors exist in dimensions 4, 5, 6 and 8; not in 7.[35]
When rolling two standard six-sided dice, seven has a 1 in 6 probability of being rolled, the greatest of any number.[37] The opposite sides of a standard six-sided die always add to 7.
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.[42]
In Western culture, seven is consistently listed as people's favorite number[43][44]
When guessing numbers 1–10, the number 7 is most likely to be picked[45]
Seven-year itch, a term that suggests that happiness in a marriage declines after around seven years
Classical antiquity
The Pythagoreans invested particular numbers with unique spiritual properties. The number seven was considered to be particularly interesting because it consisted of the union of the physical (number 4) with the spiritual (number 3).[46] In Pythagorean numerology the number 7 means spirituality.
References from classical antiquity to the number seven include:
The number seven had mystical and religious significance in Mesopotamian culture by the 22nd century BCE at the latest. This was likely because in the Sumerian sexagesimal number system, dividing by seven was the first division which resulted in infinitely repeating fractions.[51]
↑ Carl B. Boyer, A History of Mathematics (1968) p.52, 2nd edn.
↑ Georges Ifrah, The Universal History of Numbers: From Prehistory to the Invention of the Computer transl. David Bellos et al. London: The Harvill Press (1998): 395, Fig. 24.67
↑ "Μαθηματικά Α' Δημοτικού"[Mathematics for the First Grade](PDF) (in Greek). Ministry of Education, Research, and Religions. p.33. Retrieved May 7, 2018.
"...It will thus be found that, including the employment of the same figures, there are seventeen different combinations of regular polygons by which this may be effected; namely, —
42 (forty-two) is the natural number that follows 41 and precedes 43.
10 (ten) is the even natural number following 9 and preceding 11. Ten is the base of the decimal numeral system, the most common system of denoting numbers in both spoken and written language.
12 (twelve) is the natural number following 11 and preceding 13. Twelve is a superior highly composite number, divisible by the numbers from 1 to 4, and 6.
20 (twenty) is the natural number following 19 and preceding 21.
17 (seventeen) is the natural number following 16 and preceding 18. It is a prime number.
19 (nineteen) is the natural number following 18 and preceding 20. It is a prime number.
23 (twenty-three) is the natural number following 22 and preceding 24.
29 (twenty-nine) is the natural number following 28 and preceding 30. It is a prime number.
27 is the natural number following 26 and preceding 28.
84 (eighty-four) is the natural number following 83 and preceding 85. It is seven dozens.
32 (thirty-two) is the natural number following 31 and preceding 33.
31 (thirty-one) is the natural number following 30 and preceding 32. It is a prime number.
37 (thirty-seven) is the natural number following 36 and preceding 38.
57 (fifty-seven) is the natural number following 56 and preceding 58.
61 (sixty-one) is the natural number following 60 and preceding 62.
63 (sixtboo-three) is the natural number following sixtboo and preceding seventboo.
109 is the natural number following 108 and preceding 110.
144 is the natural number following 143 and preceding 145.
5 (five) is a number, numeral and digit. It is the natural number, and cardinal number, following 4 and preceding 6, and is a prime number.
14 (fourteen) is the natural number following 13 and preceding 15.
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