## Frequent Links

# Fortunate number

Are any Fortunate numbers composite? (Fortune's conjecture) |

A **Fortunate number**, named after Reo Fortune, for a given positive integer *n* is the smallest integer *m* > 1 such that *p*_{n}# + *m* is a prime number, where the primorial *p*_{n}# is the product of the first *n* prime numbers.

For example, to find the seventh Fortunate number, one would first calculate the product of the first seven primes (2, 3, 5, 7, 11, 13 and 17), which is 510510. Adding 2 to that gives another even number, while adding 3 would give another multiple of 3. One would similarly rule out the integers up to 18. Adding 19, however, gives 510529, which is prime. Hence 19 is a Fortunate number. The Fortunate number for *p*_{n}# is always above *p*_{n}. This is because *p*_{n}#, and thus *p*_{n}# + *m*, is divisible by the prime factors of *m* for *m* = 2 to *p*_{n}.

The Fortunate numbers for the first primorials are:

- 3, 5, 7, 13, 23, 17, 19, 23, 37, 61, 67, 61, 71, 47, 107, 59, 61, 109, etc. (sequence A005235 in OEIS).

The Fortunate numbers sorted in numerical order with duplicates removed:

- 3, 5, 7, 13, 17, 19, 23, 37, 47, 59, 61, 67, 71, 79, 89, 101, 103, 107, 109, 127, 151, 157, 163, 167, 191, 197, 199, ... ((sequence A046066 in OEIS)).

Reo Fortune conjectured that no Fortunate number is composite (*Fortune's conjecture*).^{[1]} A **Fortunate prime** is a Fortunate number which is also a prime number. As of 2012^{[update]}, all the known Fortunate numbers are prime.

## References

- ↑ Guy, Richard K. (1994).
*Unsolved problems in number theory*(2nd ed.). Springer. pp. 7–8. ISBN 0-387-94289-0.

- Chris Caldwell, "The Prime Glossary: Fortunate number" at the Prime Pages.
- Weisstein, Eric W., "Fortunate Prime",
*MathWorld*.