Find nine different integers from 1 to 20 inclusive such that no combination of any three of the nine integers form an arithmetic sequence.

(For example, if two of the integers chosen were 7 and 13, then that would preclude 1, 10 and 19 from being included.)

(In reply to re(2): Counting Up by Charlie)

There is something to be said for Bryan's method.  I've been trying to work it into a problem for the past week - independently of this problem.

I've not considered how Compact a set of n integers can be with no 3 in arithmetic sequence.  But I can see by scanning your list that C(9)=20.

It looks like C(7)=13 and  C(8) = 14 etc but I'm not sure enough about what your program is doing to know if the first line of solution are minimums.

EDIT:  I could have looked it up myself.  http://oeis.org/A065825

Edited on April 9, 2013, 12:49 am

Posted by Jer on 2013-04-09 00:46:52