In the
beginning, Indians wrote 7 more or less in one stroke as a curve that looks like an uppercase ⟨J⟩ vertically inverted (ᒉ). The western Ghubar Arabs' 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
Arabs 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 12th century 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 the two 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]
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.
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).[29][30] This is also one of twenty-one such configurations from seventeen combinations of polygons, that features the largest and smallest polygons possible.[31][32]
Otherwise, for any regular n-sided polygon, the maximum number of intersecting diagonals (other than through its center) is at most 7.[33]
Seven of eight
semiregular tilings are Wythoffian (the only exception is the
elongated triangular tiling), where there exist three tilings that are
regular, all of which are Wythoffian.[35] Seven of nine uniform colorings of the square tiling are also Wythoffian, and between the
triangular tiling and
square tiling, there are seven non-Wythoffian uniform colorings of a total twenty-one that belong to regular tilings (all
hexagonal tiling uniform colorings are Wythoffian).[36]
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.[37][38]
Also, the lowest known dimension for an
exotic sphere is the seventh dimension, with a total of 28 differentiable structures; there may exist exotic smooth structures on the
four-dimensional sphere.[49][50]
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.[51] On the other hand, such figures with rank n + 3 mirrors exist in dimensions 4, 5, 6 and 8; not in 7.[52] Hypercompact polytopes with lowest possible rank of n + 2 mirrors exist up through the 17th dimension, where there is a single solution as well.[53]
The positive
definite quadraticinteger matrix representative of all
odd numbers contains the set of seven integers: {1, 3, 5, 7, 11, 15, 33} where seven is the middle indexed member.[55][56]
When rolling two standard six-sided
dice, seven has a 6 in 62 (or 1/6) probability of being rolled (1–6, 6–1, 2–5, 5–2, 3–4, or 4–3), the greatest of any number.[57] The opposite sides of a standard six-sided dice 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.[60] For example, 1/7 = 0.142857 142857... and 2/7 = 0.285714 285714....
In fact, if one sorts the digits in the number 142,857 in ascending order, 124578, it is possible to know from which of the digits the decimal part of the number is going to begin with. The remainder of dividing any number by 7 will give the position in the sequence 124578 that the decimal part of the resulting number will start. For example, 628 ÷ 7 = 89+5/7; here 5 is the remainder, and would correspond to number 7 in the ranking of the ascending sequence. So in this case, 628 ÷ 7 = 89.714285. Another example, 5238 ÷ 7 = 748+2/7, hence the remainder is 2, and this corresponds to number 2 in the sequence. In this case, 5238 ÷ 7 = 748.285714.
In Western culture, seven is consistently listed as people's favorite number[61][62]
When guessing numbers 1–10, the number 7 is most likely to be picked[63]
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).[64] In Pythagorean
numerology the number 7 means spirituality.
References from classical antiquity to the number seven include:
Other references to the number seven in traditions from around the world 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.[68]
^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, —