A102642 a(n) = A102640(n) + A102641(n) - 1.

2, 3, 5, 5, 4, 5, 3, 4, 4, 5, 3, 7, 3, 6, 3, 3, 4, 6, 3, 6, 4, 7, 3, 6, 3, 5, 3, 5, 4, 7, 3, 5, 4, 5, 5, 7, 3, 6, 5, 5, 3, 6, 3, 6, 5, 5, 3, 7, 3, 5, 4, 5, 3, 6, 7, 3, 6, 3, 3, 4, 3, 5, 4, 3, 4, 6, 3, 5, 5, 6, 3, 7, 3, 5, 7, 5, 5, 6, 3, 5, 6, 5, 3, 4, 3, 5, 3, 6, 3, 5, 3, 5, 4, 5, 5, 5, 4, 6, 4, 7, 3, 6, 3, 4, 7

Offset

1,1

Comments

A006530(2^n)=2 is a local minimum. Actual sequence displays the "width of valley" between the two nearest peaks of largest prime divisors. At the bottom of valley lies the number 2, the minimum.

Links

Table of n, a(n) for n=1..105.

Example

n=12: 2^10=4096. The greatest prime divisors of numbers around 4096 [both downward and upward] are as follows: {31, 4093, 89, 13, 2, 241, 683, 4099, 41}. The length of relevant sequence, i.e., between peaks 4093 and 4099 is 7, thus a(12)=7.

Mathematica

With[{nn = 12, lim = 105}, Map[Total@ # - 1 &, Transpose@ {Table[Function[k, 1 + LengthWhile[#, # > 0 &] &@ Differences@ Array[FactorInteger[#][[-1, 1]] &, nn, k]][2^n], {n, lim}], Table[Function[k, 1 + LengthWhile[#, # > 0 &] &@ Differences@ Table[FactorInteger[m][[-1, 1]], {m, k, k - nn, -1}]][2^n], {n, lim}]}]] (* Michael De Vlieger, Jul 30 2017 *)

Crossrefs

Cf. A006530, A102640, A102641, A102643, A102644.

Sequence in context: A096289 A367848 A131295 * A183228 A133304 A246401

Adjacent sequences: A102639 A102640 A102641 * A102643 A102644 A102645

Keyword

nonn

Author

Labos Elemer, Jan 21 2005

Status

approved