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7 is the fourth prime number and the fourth single-digit prime. The prime factors of 7 are 1 and 7. It is said to be a lucky number in some mythologies, and 7 is a winning number in some games of chance, e.g. a lotto.

7 is the second Mersenne Prime (since 23-1=7) and a Double Mersenne Prime (since 3 is also a Mersenne Prime), as well as the first Carol Prime.

## Proofs

• Therefore 7 has 2 factors

## As an Exponent of Mersenne Number

27 - 1, which is the number 127, is a prime number. It is the 4th Mersenne Prime.

Since 7 is a Mersenne Prime, 127, being 223-1-1, is a Double Mersenne Prime.

## Relationship with other odd numbers

### The numbers before

• The previous odd number, 5, is a prime number.
• There are no odd numbers between 5 and 7. As a result, they are twin primes.
• The odd number previous to 5 is 3. 3 is a prime number.
• 3 and 7 are four numbers apart; therefore, they are cousin primes.

### The numbers after

• The next odd number, 9, is a square number. It is the first odd composite number.
• 11, the odd number following 9, is a prime number.
• 7 and 11 are four numbers apart, therefore, they are cousin primes.
• 13, the odd number following 11, is a prime number.
• 7 and 13 are six numbers apart; therefore, they are sexy primes.

## Trivia

• 7 is the only number that is part of both cousin prime pairs, (3, 7) and (7, 11).
• In Final Fantasy VII, there's an easter egg called All Lucky 7s, where if the character has 7777 HP, (s)he will enter the state, launching a barrage of 63 hits, each of them doing 7777 damage.
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