>>16715658But (2) says m greater than or equal to 3, not just equal.
I got the same answer for m = 3, but the goal is to generalise it for m = 4, 5, 99, so on.
So, we add another 4 terms to a, to form one extra subsequence, none of which must contain element 2 nor 13, yet must be evenly spaced, by m distance.
Hence, the numbers (n) can be separated modulo m: n mod m, n+1 mod m, ... m-1 mod m, and each 4-length subsequence will be entirely contained within one of these divisions.
If 2 or 13 are in one of these divisions, they are skipped. To prove this, for m >= 3: 2 and 13 must be the first or (modolu 4) first element, so a gap can be left without sequences breaking an arithmetic sequence. 2 is a prime number, and as 2 < 3, must always be the first, so QED. 13 is also a prime, and 13 = 4*3+1 = 1 (modulo 4) so also QED.
So, m = 3, the arithmetic sequence a1 to a(4m+2) is 2,13-separable.
And, if m - 1 is a 2,13 separable sequence, then so is m,
Therefore, all sequences m >= 3 are (2,13)-separable.