7th Marine Brigade (Reserve)

7 (seven) is the natural number following 6 and preceding 8. It is the only prime number preceding a cube.

As an early prime number in the series of positive integers, the number seven has greatly symbolic associations in religion, mythology, superstition and philosophy. The seven Classical planets resulted in seven being the number of days in a week. It is often considered lucky in Western culture and is often seen as highly symbolic. Unlike Western culture, in Vietnamese culture, the number seven is sometimes considered unlucky.

It is the first natural number whose pronunciation contains more than one syllable.

Evolution of the Arabic digit

In the beginning, Indians wrote 7 more or less in one stroke as a curve that looks like an uppercase 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 digit more rectilinear. The eastern Arabs developed the digit from a form that looked something like our 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. 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 the seven-segment displays of pocket calculators and digital watches, 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, and some in Britain and Ireland as well as Latin America, write 7 with a line in 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, France, Italy, Belgium, Finland, Romania, Germany, Greece, and Hungary.

Mathematics

Seven, the fourth prime number, is not only a Mersenne prime (since ) but also a double Mersenne prime since the exponent, 3, is itself a Mersenne prime. It is also a Newman–Shanks–Williams prime, a Woodall prime, a factorial prime, a lucky prime, a happy number (happy prime), a safe prime (the only ), a Leyland prime of the second kind and the fourth Heegner number.
*Seven is the lowest natural number that cannot be represented as the sum of the squares of three integers. (See Lagrange's four-square theorem#Historical development.)
*Seven is the aliquot sum of one number, the cubic number and is the base of the 7-aliquot tree.
*7 is the only number ''D'' for which the equation has more than two solutions for ''n'' and ''x'' natural. In particular, the equation is known as the Ramanujan–Nagell equation.
*7 is the only dimension, besides the familiar 3, in which a vector cross product can be defined.
*7 is the lowest dimension of a known exotic sphere, although there may exist as yet unknown exotic smooth structures on the 4-dimensional sphere.
* divided by 7 is exactly . 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, and
: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 = ; here 5 is the remainder, and would correspond to number 7 in the ranking of the ascending sequence. So in this case, . Another example, , hence the remainder is 2, and this corresponds to number 2 in the sequence. In this case, .
*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, seven has a 6 in 6 (or ) probability of being rolled (1–6, 6–1, 2–5, 5–2, 3–4, or 4–3), the greatest of any number. The opposite sides of a standard six-sided dice always add to 7.
*The Millennium Prize Problems are seven problems in mathematics that were stated by the Clay Mathematics Institute in 2000. Currently, six of the problems remain unsolved.

Basic calculations

In science

* Seven colors in a rainbow: ROYGBIV
* Seven Continents
* Seven Seas
* Seven climes
* The neutral pH balance
* Number of music notes in a scale
* Number of spots most commonly found on ladybugs
* Atomic number for Nitrogen

In Psychology

* Seven, plus or minus two as a model of working memory.
* Seven psychological types called the Seven Rays in the teachings of Alice A. Bailey
* In Western Culture, Seven is consistently listed as people's favorite number.
* When guessing numbers 1-10 the number 7 is most likely to be picked.
* Seven-year itch: happiness in marriage said to decline after 7 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). In Pythagorean numerology the number 7 means spirituality.

References from classical antiquity to the number seven include:

*Seven Classical planets and the derivative Seven Heavens
* Seven Wonders of the ancient world
*Seven metals of antiquity
* Seven days in the week
* Seven Seas
* Seven Sages
* Seven champions that fought Thebes
* Seven hills of Rome and Seven Kings of Rome
* Seven Sisters, the daughters of Atlas also known as the Pleiades

Religion and mythology

Judaism

The number seven forms a widespread typological pattern within Hebrew scripture, including:
*Seven days (more precisely yom) of Creation, leading to the seventh day or Sabbath (Genesis 1)
*Seven-fold vengeance visited on upon Cain for the killing of Abel (Genesis 4:15)
*Seven pairs of every clean animal loaded onto the ark by Noah (Genesis 7:2)
*Seven years of plenty and seven years of famine in Pharaoh's dream (Genesis 41)
*Seventh son of Jacob, Gad, whose name means good luck (Genesis 46:16)
*Seven times bullock's blood is sprinkled before God (Leviticus 4:6)
*Seven nations God told the Israelites they would displace when they entered the land of Israel (Deuteronomy 7:1)
*Seven days of the Passover feast (Exodus 13:3–10)
*Seven-branched candelabrum or Menorah (Exodus 25)
*Seven trumpets played by seven priests for seven days to bring down the walls of Jericho (Joshua 6:8)
*Seven things that are detestable to God (Proverbs 6:16–19)
*Seven Pillars of the House of Wisdom (Proverbs 9:1)
*Seven archangels in the deuterocanonical Book of Tobit (12:15)

References to the number seven in Jewish knowledge and practice include:
*Seven divisions of the weekly readings or aliyah of the Torah
*Seven Jewish men (over the age of 13) called to read aliyahs in Shabbat morning services
*Seven blessings recited under the chuppah during a Jewish wedding ceremony
*Seven days of festive meals for a Jewish bride and groom after their wedding, known as Sheva Berachot or Seven Blessings
*Seven Ushpizzin prayers to the Jewish patriarchs during the holiday of Sukkot

Christianity

Following the traditional of the Hebrew Bible, the New Testament likewise uses the number seven as part of a typological pattern:

*Seven loaves multiplied into seven basketfuls of surplus (Matthew 15:32–37)
*Seven demons were driven out of Mary Magdalene (Luke 8:2)
* Seven last sayings of Jesus on the cross
*Seven men of honest report, full of the Holy Ghost and wisdom (Acts 6:3)
* Seven Spirits of God, Seven Churches and Seven Seals in the Book of Revelation

References to the number seven in Christian knowledge and practice include:
* Seven Gifts of the Holy Spirit
* Seven Corporal Acts of Mercy and Seven Spiritual Acts of Mercy
* Seven deadly sins: lust, gluttony, greed, sloth, wrath, envy, and pride, and seven terraces of Mount Purgatory
* Seven Virtues: chastity, temperance, charity, diligence, kindness, patience, and humility
* Seven Joys and Seven Sorrows of the Virgin Mary
* Seven Sleepers of Christian myth
*Seven Sacraments in the Catholic Church (though some traditions assign a different number)

Islam

References to the number seven in Islamic knowledge and practice include:
*Seven ayat in surat al-Fatiha, the first book of the holy Qur'an
*Seven circumambulations of Muslim pilgrims around the Kaaba in Mecca during the Hajj and the Umrah
*Seven walks between Al-Safa and Al-Marwah performed Muslim pilgrims during the Hajj and the Umrah
*Seven doors to hell (for heaven the number of doors is eight)
*Seventh day naming ceremony held for babies
*Seven enunciators of divine revelation (''nāṭiqs'') according to the celebrated Fatimid Ismaili dignitary Nasir Khusraw
* Circle Seven Koran, the holy scripture of the Moorish Science Temple of America

Hinduism

References to the number seven in Hindu knowledge and practice include:
*Seven worlds in the universe and seven seas in the world in Hindu cosmology
*Seven sages or Saptarishi and their seven wives or Sapta Matrka in Hindu mythology
*Seven Chakras in eastern philosophy
*Seven stars in a constellation called " Saptharishi Mandalam" in Indian astronomy
*Seven promises, or Saptapadi, and seven circumambulations around a fire at Hindu weddings
*Seven virgin goddesses or Saptha Kannimar worshipped in temples in Tamil Nadu, India
*Seven hills at Tirumala known as Yedu Kondalavadu in Telugu, or ezhu malaiyan in Tamil, meaning "Sevenhills God"
*Seven steps taken by the Buddha at birth
*Seven divine ancestresses of humankind in Khasi mythology
*Seven octets or Saptak Swaras in Indian Music as the basis for Ragas compositions
* Seven Social Sins listed by Mahatma Gandhi

Eastern tradition

Other references to the number seven in Eastern traditions include:

* Seven Lucky Gods or gods of good fortune