A130904 Numbers n such that the trajectory of the map n -> (n + lpf(n)) / 2 reaches 3, where lpf(n) is the least prime factor of n (A020639).

3, 4, 6, 9, 10, 15, 16, 18, 25, 27, 28, 30, 33, 34, 48, 49, 51, 52, 54, 55, 57, 58, 63, 64, 66, 91, 93, 94, 96, 99, 100, 102, 105, 106, 108, 111, 112, 114, 119, 121, 123, 124, 126, 129, 130, 169, 180, 183, 184, 186, 187, 189, 190, 195, 196, 198

Offset

1,1

Comments

The only prime term is 3 because all primes map to themselves.

Links

Table of n, a(n) for n=1..56.

Example

(15 + 3) / 2 = 9, (9 + 3) / 2 = 6, (6 + 2) / 2 = 4, and (4 + 2) / 2 = 3, thus 15 is in the sequence.
(21 + 3) / 2 = 12, (12 + 2) / 2 = 7, and (7 + 7) / 2 = 7, so 21 never reaches 3 and therefore is not in the sequence.

Crossrefs

Cf. A177980, A020639, A061228.

Sequence in context: A005122 A166161 A368136 * A034706 A245810 A054686

Adjacent sequences: A130901 A130902 A130903 * A130905 A130906 A130907

Keyword

nonn

Author

Grant Garcia, Jan 06 2011

Status

approved