# 7 (number)

natural number
(Redirected from Seven)

The number seven is a natural number that comes after six and before eight. In Roman numerals, it is VII.

 ← 6 7 8 →
Cardinalseven
Ordinal7th
(seventh)
Numeral systemseptenary
Factorizationprime
Prime4th
Divisors1, 7
Greek numeralΖ´
Roman numeralVII
Roman numeral (unicode)Ⅶ, ⅶ
Greek prefixhepta-/hept-
Latin prefixseptua-
Binary1112
Ternary213
Quaternary134
Quinary125
Senary116
Octal78
Duodecimal712
Vigesimal720
Base 36736
Greek numeralZ, ζ
Amharic
Arabic٧,7
Germanic Variant7
Persian٧ - هفت
Urdu Bengali
Chinese numeral七(qi)
Devanāgarī (saat)
Telugu
Tamil
Hebrewז (Zayin)
Khmer
Thai
Pronunciation of the number 7

## Mathematics

In mathematics, the number seven is an odd number and a prime number. It is also a Mersenne prime.

### Divisibility Rule

• To test if an integer is divisible by 7 or not,
1. Take off the last digit and multiply it by 2.
2. Subtract that product from the root number (the integer without the last digit).
3. Repeat the second step again and again until it is clearly that the number is divisible by 7 or not.

Example

918785

1. 5×2=10, 91878-10=91868
2. 8×2=16, 9186-16=9170
3. 0×2=0, 917-0=917
4. 7×2=14, 91-14=77

77 is divisible by 7, so the original number 918785 is also divisible by 7.

Proof: Pretend $a$  is the root number and $b$  is the last digit. Then $10a+b$  is the whole number. If the whole number ($10a+b$ ) is divisible by 7, $3a+b$  is also a multiple of 7. And $(3a+b)-7b=3a-6b$  is also a multiple 7. That proves $3a-6b$  is divisible by seven, since 3 and 6 are both multiples of 3. So $a-2b$  is also divisible by 7.

## In the world

• There are seven colors in the rainbow.
• The atomic number of nitrogen.
• The number of horizontal rows of elements in the periodic table.
• Seven gates of heaven (QUR'AN 15th chapter AL-HIJR verse 44)915:44)
• Seven days in a week
• Seven seas and seven continents.
• Seven major parts of the human body (2 legs, 2 hands, trunk, neck and head)