# Lucky number

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A lucky number is a natural number in a set which is generated by a "sieve" similar to the Sieve of Eratosthenes that generates the primes.

We begin with a list of integers starting with 1:

```1, 2, 3, 4, 5, 6, 7, 8, 9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,
```

Then we cross out every second number (all even numbers), leaving only the odd integers:

```1,    3,    5,    7,    9,   11,   13,   15,   17,   19,   21,   23,   25,
```

The second term in this sequence is 3. Now we cross out every third number which remains in the list:

```1,    3,          7,    9,         13,   15,         19,   21,         25,
```

The third surviving number is now 7 so we cross out every seventh number that remains:

```1,    3,          7,    9,         13,   15,               21,         25,
```

If we repeat this procedure indefinitely, the survivors are the lucky numbers:

1, 3, 7, 9, 13, 15, 21, 25, 31, 33, 37, 43, 49, 51, 63, 67, 69, 73, 75, 79, 87, 93, 99, ...

Stanisław Ulam was the first to discuss these numbers, around 1955. He named them "lucky" because of a connection with a story told by the historian Josephus.

Lucky numbers share some properties with primes, such as asymptotic behaviour according to the prime number theorem; also Goldbach's conjecture has been extended to them. There are infinitely many lucky numbers. It is not known whether there are also infinitely many lucky primes:

3, 7, 13, 31, 37, 43, 67, 73, 79, 127, 151, 163, 193, ...