7 (number)

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

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

• 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).^[1]
3. Repeat the second step again and again until it is clear 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 ${\displaystyle a}$ is the root number and ${\displaystyle b}$ is the last digit. Then ${\displaystyle 10a+b}$ is the whole number. If the whole number (${\displaystyle 10a+b}$) is divisible by 7, ${\displaystyle 3a+b}$ is also a multiple of 7. And ${\displaystyle (3a+b)-7b=3a-6b}$ is also a multiple 7. That proves ${\displaystyle 3a-6b}$ is divisible by seven, since 3 and 6 are both multiples of 3. So ${\displaystyle a-2b}$ is also divisible by 7.

• There are seven colours in the rainbow.
• The atomic number of nitrogen.
• The number of horizontal rows of elements in the periodic table.
• The seven sacraments of the Catholic church.
• The number of completion and fullness (HOLY BIBLE - Genesis 1; 2:1-2 - God is described to have created the Heavens and the Earth in six days, and rested on the seventh).
• Seven gates of heaven (QUR'AN - 15th chapter AL-HIJR verse 44)915:44).
• Seven days in a week.
• Seven seas and seven continents.
• The Wonders of the World (seven each in different segments of our world, such as cities, nature, underwater, and space)
• Portuguese football player Cristiano Ronaldo wears the number 7, widely known as CR7.
• “007”, is the codename for James Bond, a fictional MI6 spy (the “00” prefix signifying a “license to kill” and the “7” identifying Bond as the seventh agent in the 00 section of MI6).
• The seven dwarfs in Walt Disney’s ‘Snow White and the Seven Dwarfs’ (1937).
• The number 7 is often considered as luck in Western culture.