|
|
A161812
|
|
Numbers with decreasing but not strictly decreasing prime signature.
|
|
2
|
|
|
6, 30, 36, 60, 120, 180, 210, 216, 240, 420, 480, 840, 900, 960, 1080, 1260, 1296, 1680, 1800, 1920, 2310, 2520, 3360, 3600, 3840, 4620, 5040, 5400, 6300, 6480, 6720, 7200, 7560, 7680, 7776, 9240, 10080, 12600, 13440, 13860, 14400, 15120, 15360, 18480
(list;
graph;
refs;
listen;
history;
text;
internal format)
|
|
|
OFFSET
|
1,1
|
|
COMMENTS
|
All numbers of the form 2^k1*3^k2*...*p_n^k_n, where k1 >= k2 >= ... >= k_n, and the '=' occurs at least once, sorted.
It appears that all highly composite numbers (A002182) greater than 720 are in this sequence. - Walter Roscello, Dec 24 2013
|
|
LINKS
|
|
|
EXAMPLE
|
a(1) = 2*3, a(2) = 2*3*5, a(3) = 2^2*3^2, a(4) = 2^2*3*5.
|
|
MATHEMATICA
|
okQ[n_] := Module[{pp, ee}, {pp, ee} = Transpose[FactorInteger[n]]; Length[pp] == PrimePi[pp // Last] && GreaterEqual @@ ee && Sort[ee] != Union[ee]]; Select[Range[20000], okQ] (* Jean-François Alcover, Jun 17 2019 *)
|
|
CROSSREFS
|
|
|
KEYWORD
|
easy,nonn
|
|
AUTHOR
|
|
|
EXTENSIONS
|
|
|
STATUS
|
approved
|
|
|
|