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 Counting Up by Bryan)

Such a progression seems to yield only 8 integers:
1,2, 4,5, 10,11, 13,14
the next number that avoids an arithmetic sequence is 28.

Posted by Jer on 2013-04-08 16:01:02