OFFSET
1,1
COMMENTS
The asymptotic density of this sequence is 1 - Product_{p prime} (1-1/p^(p+1)) = 0.13585792767780221591... . - Amiram Eldar, Feb 14 2023
Verified up to a(120) = 1000, except for a(16) = 162 and a(55) = 486, every a(n) is also the order of an isomorphism class for which there exists at least one nonabelian nilpotent group G such that |Aut(G)|/a(n) is nonintegral. Within the same range there are 26 group orders not in a(n), which, except for 3^4*2^3 = 648, all have the form 3^3*m or 5^3*k, with m and k being prime, squarefree, or nonsquarefree. - Miles Englezou, Jul 16 2024
LINKS
Michel Marcus, Table of n, a(n) for n = 1..10000
EXAMPLE
48 = 2^4*3^1 is a term but 12 = 2^2*3^1 is not.
MATHEMATICA
Select[Range[500], AnyTrue[(f = FactorInteger[#]), First[#1] > Last[#1] &] && AnyTrue[f, First[#1] < Last[#1] &] &] (* Amiram Eldar, Nov 13 2020 *)
PROG
(PARI) isok(n) = my(f=factor(n), b1=0, b2=0); for (i=1, #f~, if (f[i, 1] < f[i, 2], b1=1, if (f[i, 1] > f[i, 2], b2=1))); return(b1 && b2); \\ Michel Marcus, Nov 13 2020
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
EXTENSIONS
More terms from Reiner Martin, Jul 07 2001
STATUS
approved