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# Seven

Seven (7) is the natural number following six and preceding eight.

Seven is the fourth-smallest prime number; the next is eleven. Seven is a Mersenne prime. This is because 23-1=7, and seven is a prime number.

7 goes into 999,999 exactly 142,857 times, so fractions with 7 in the denominator have six-digit repeating sequences in their decimal expansions. 1/7=0.142857142...

In binary code seven is 111; in ternary code seven is 21; in quaternary numeral system code seven is 13; in quinary seven is 12; in senary seven is 11; in septenary seven is 10; in octal code and all codes above (such as decimal and hexadecimal) seven is 7. In Roman numerals seven is VII.

A seven-sided polygon is a heptagon. The regular n-gons for n ≤ 6 can be constructed by ruler and compass alone, but the regular heptagon cannot.

There are seven of musicians in a septet and seven babies born in a set of septuplets. The most famous set of septuplets are the McCaughey Septuplets, who was born in 1997.

The British fifty-pence coin is a heptagon, with the sides curved to give it a constant radius.

Cities constructed amidst seven hills include:

Stephen Covey wrote a book called The Seven Habits of Highly Successful People.

Seven is also:

• the traditional number of Wonders of the Ancient World.
• viewed as a lucky number in many Western cultures, and in Japanese culture. In Western cultures it is considered lucky because the numbers three and four added together equal 7. Those two numbers are lucky as well, with 7 being extra lucky.
• the number of days in a week.
• a highly symbolic number in Christian thought (being the number of days in which God created the earth, etc)
• referred to in bingo as 'Lucky Seven'.
• the number of the Deadly Sins: lust, avarice, envy, pride, sloth, gluttony and anger.
• considered the usual number for a witches' coven.
• the neutral pH level.
• the number of palms in an Egyptian Sacred Cubit.
• the number of the ages of man into which William Shakespeare divided a lifetime.
• the name of one of the monsters in the cartoon television show Seven Little Monsters.
• the figurative number of seas.
• the number of candles in the Jewish menorah.
• the number of colors of the rainbow.
• the number of basic principles of the bushido.
• the number of heads of the beast, and of some other monsters, like the hydra.
• the number of points on a sheriff's star.
• the number of frieze groups, the groupss consisting of symmetries of the plane whose group of translationss is isomorphic to the group of integers
• the code for international direct-dialed phone calls to Russia and Kazakhstan.

and occurs in the names of: