The first column comprises consecutive cubes starting with 3. The second looks like months of the year, starting with January, then March (or May), then ? then August and December. Since the first two entries are not represented by cubes, the sequence would look like, say, 0,0,1,3,5,8,12: A000212 in Sloane.
The function n^3 => Floor(n^2/3) produces the result that the missing letter is M.
possible solution
