7 (number)

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[edit] In mathematics

  1. Remove the last digit,
  2. Double it, and
  3. Subtract it from the remaining digits.
  4. If the result is negative and there are 2 or more digits, drop the negative sign.
  5. Repeat until you end up with a result that is a multiple of seven (7). (i.e. -7, 0, or +7)
For example, the number 1358 is evenly divisible by seven, since:
135 - (8*2) = 119
11 - (9*2) = -7
Using Number Theory the proof is rather easy, once we rewrite the number n in the form:
n = 10a + b
Where:
a is the remaining digits, and
b is the last digit.
Then:
10a + b = 0 (mod 7)
5 * (10a + b) = 0 (mod 7)
49a + a + 5b = 0 (mod 7)
a + 5b - 7b = 0 (mod 7)
a - 2b = 0 (mod 7)

[edit] 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, etc) 7

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

[edit] List of basic calculations

Multiplication 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 50 100 1000
7 \times x 7 14 21 28 35 42 49 56 63 70 77 84 91 98 105 112 119 126 133 140 147 154 161 168 175 350 700 7000
Division 1 2 3 4 5 6 7 8 9 10
11 12 13 14 15
7 \div x 7 3.5 2.\overline{3} 1.75 1.4 1.1\overline{6} 1 0.875 0.\overline{7} 0.7
0.\overline{6}\overline{3} 0.58\overline{3} 0.\overline{5}3846\overline{1} 0.5 0.4\overline{6}
x \div 7 0.\overline{1}4285\overline{7} 0.\overline{2}8571\overline{4} 0.\overline{4}2857\overline{1} 0.\overline{5}7142\overline{8} 0.\overline{7}1428\overline{5} 0.\overline{8}5714\overline{2} 1 1.\overline{1}4285\overline{7} 1.\overline{2}8571\overline{4} 1.\overline{4}2857\overline{1}
1.\overline{5}7142\overline{8} 1.\overline{7}1428\overline{5} 1.\overline{8}5714\overline{2} 2 2.\overline{1}4285\overline{7}
Exponentiation 1 2 3 4 5 6 7 8 9 10 11 12 13
7 ^ x\, 7 49 343 2401 16807 117649 823543 5764801 40353607 282475249 1977326743 13841287201 96889010407
x ^ 7\, 1 128 2187 16384 78125 279936 823543 2097152 4782969 10000000 19487171 35831808 62748517
Radix 1 5 10 15 20 25 30 40 50 60 70 80 90 100
110 120 130 140 150 200 250 500 1000 10000 100000 1000000
x_{7} \ 1 5 13_{7} \ 21_{7} \ 26_{7} \ 31_{8} \ 36_{8} \ 50_{8} \ 62_{8} \ 74_{8} \ 106_{8} \ 120_{8} \ 132_{8} \ 144_{8} \
156_{8} \ 170_{8} \ 202_{8} \ 214_{8} \ 226_{8} \ 310_{8} \ 372_{8} \ 764_{8} \ 1750_{8} \ 23420_{8} \ 303240_{8} \ 3641100_{8} \

[edit] Evolution of the glyph

Image:SevenGlyph.svg

In the beginning, various Hindus 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. 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 7 is written as in ① in Japan, Korea and so on.

In fonts with text figures, 7 usually has a descender, for example, Image:TextFigs078.png.

[edit] In science

  • The number of spots in a common ladybug.

[edit] Chemistry

[edit] Astronomy

[edit] In technology

[edit] In religion

  • In Judaism:
    • A highly symbolic number in the Torah, alluding to the infusion of spirituality and Godliness into the Creation. For example:
    1. God rested on and sanctified the 7th day (Shabbat)
    2. A seven-day purification period is required for one who has become tamei to become tahor
    3. The Shemittah (Sabbatical) year arrives every 7th year
    4. The Jubilee (Yovel) year comes after 7 times 7 years
    5. The Counting of the Omer leading up to the Giving of the Torah is expressed as "7 times 7 weeks"
    • The weekly Torah portion is divided into seven aliyahs, and seven men or boys over the age of 13 are called up for the reading of these aliyahs during Shabbat morning services
    • Seven blessings are recited under the chuppah during a Jewish wedding ceremony
    • David was the seventh son of Jesse
    • A Jewish bride and groom are feted with seven days of festive meals after their wedding, known as Sheva Berachot ("Seven Blessings")
    • The number of Ushpizzin (also known as the "Seven Shepherds") who visit the sukkah during the holiday of Sukkot: Abraham, Isaac, Jacob, Joseph, Moses, Aaron, and David
    • The number of nations God told the Israelites they would displace when they entered the land of Israel (Deut. 7:1): the Hittite, the Girgashite, the Amorite, the Canaanite, the Perizzite, the Hivite, and the Jebusite
    • In Breslov tradition, the seven orifices of the face (2 eyes, 2 nostrils, 2 ears, and the mouth) are called "The Seven Candles"
  • In Islam:
    • The number of ayat in surat al-Fatiha
    • The number of heavens in Islamic tradition
    • The number of Earths in Islamic tradition
    • The numbers 7, 70, 70,000, etc. are used in Islamic traditions to denote an infinite or high number. In Arabic, 70, 70,000, etc. is used to mean infinite. This is because 1 is the smallest number, 2 is just after 1, 5 and 10 are exact, 4 and 6 are just before and after 5, and 9 is just before 10. This leaves 3, 7, and 8. 8 is closer to the end although it is larger. 8 is also an even number, so divisible by 2. Between 3 and 7, 3 is smaller so 7 is chosen to represent infinite.
  • In Christianity:
  • The number of times Cain will be avenged by God if he is murdered. Gen 4:15
  • In the Bible, the number seven is the perfect number, six is imperfect.
  • The number of Archangels according to some systems. Seven Archangels
  • The minor symbol number of yang from the Taoist yin-yang
  • The number of palms in an Egyptian Sacred Cubit
  • The number of ranks in Mithraism
  • The number seven is of particular significance within Cherokee cosmology
  • In Buddhism, Buddha walked 7 steps at his birth.

[edit] In mythology

  • In Khasi mythology, the seven divine women who were left behind on earth and became the ancestresses of all humankind
  • The number of gateways traversed by Inanna during her descent into the underworld
  • The number of sleeping men in the Christian myth of the "Seven Sleepers"
  • The number of sages in Hindu mythology. Their wives are the goddesses referred to as the "Seven Mothers"
  • The number of main islands of mythological Atlantis
  • In Guaraní mythology, the number of prominant legendary monsters.
  • Seven Lucky Gods exist in Japanese mythology.

[edit] In psychology

  • The Magical Number Seven, Plus or Minus Two is a 1956 paper by cognitive psychologist George Miller who found a correlation between various human cognitive capacity limits within the range 7±2.
  • When asked to choose a number between 1 and 10, the most common number chosen appears to be 7. This choice appears to be driven by peoples idea of the properties a random number should have.

[edit] In music

Seven is part of all of the name of many musical works:

[edit] Albums

[edit] Songs

[edit] In television

[edit] In film

[edit] In sports

[edit] In other fields

  • The reason for 7 being a lucky number symbolically (such as in the bible), is because it represents the union of man and woman, since the number for women is 4 and the number for men is 3.
  • Seven candles are lit in the kinara during the African-American celebration of Kwanzaa, which is a seven-day holiday
  • It is traditionally thought that there are seven continents on Earth.
  • Seven is the smallest positive integer requiring more than one syllable 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.
  • Serial sevens are used as a diagnostic test in medicine.
  • In linguistics, the "7" is sometimes used to represent a glottal stop, as a substitute for ʔ.
  • The Harry Potter series has several references to the number seven.
  • Game developer Bungie Studios use the number 7 extensively throughout all their games. For example a character in the Halo games is called 343 Guilty Spark, 343 being 7 to the power of 3.
  • In Fist of the North Star (Hokuto no Ken), the main character, Kenshiro, is also known as "the man with seven wounds" for the seven scars on his body in the shape of the Big Dipper.
  • Year 7 is the first year of secondary education in England and Wales, equivalent to the American 6th grade.
  • Seventh grade is the seventh year of American education since kindergarten.

Seven is also:

[edit] Names and titles

Seven or 7 is all or part of a number of names and titles of people, artworks or organizations, including:

[edit] Historical years

A.D. 7, 7 B.C., 1907, 2007, etc.

[edit] Notes

  1. ^ "21 Then Peter came to Jesus and asked, 'Lord, how many times shall I forgive my brother when he sins against me? Up to seven times?' 22 Jesus answered, 'I tell you, not seven times, but seventy-seven times' ...". [1]

[edit] See also

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