Always an even number. Aside from the first term, appears to always have a last digit of 2, 4, 6 or 8. n for M3, M6, M10 and M13 are represented here, while n for M19, M21, M22 are too large for this list.
Table of n, a(n) for n=1..4.
Cf. A000668. Subsequence of A201819. Shares at least one term (0) with A224856.
Sequence in context: A203393 A235771 A235554 * A091715 A224528 A206147
Adjacent sequences: A255342 A255343 A255344 * A255346 A255347 A255348
J. W. Helkenberg, Feb 21 2015