I am looking for n consecutive integers such that (i) every number in the sequence is divisible by a prime <=n and (ii) every prime number <=n is a factor of at least two of the numbers. For example, consider n=3:
a) There are two primes less than or equal to 3. They are 2 and 3.
b) 6 7 8 does not work, in part because 7 is not evenly divisible by either 2 or 3
c) 8 9 10 does not work, even though all are divisible by 2 or 3, because 3 divides only one of them

There is some reason to believe that no sequence of positive integers works for n < 20. For n = 20:

1) What is the first sequence of 20 consecutive positive integers that works?
2) What is the second?
3) How often do they repeat after that?
4) What interesting number results if you add the first integer from one of the first two sequences to the last integer of the other?

By the way, this problem grew out of JLo's innocent perplexus problem "Six numbers and a prime"

The cases n=1..14 can be hand-verified rather easily, which I have done a while ago. n=17 can be ruled out too, because primes obviously won't work. Remain the cases n=15/16. If there was a solution for these cases, the first integer in the sequence should not be much larger than about 2*3*5*7*11*13=30030. Since Charlie already went up to a million, these cases can be ruled out too. Not sure which other n's also work.

Posted by JLo on 2006-10-16 06:17:55