7 (number)

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 colors 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.
• Seven gates of heaven (QUR'AN 15th chapter AL-HIJR verse 44)915:44).
• Seven days in a week.
• Seven seas and seven continents.
• Cristiano Ronaldo wears the number 7, widely known as CR7.
• The number 7 is often considered as luck in Western culture.