OFFSET
1,2
COMMENTS
Not a permutation of natural numbers: a(4) = a(7) = 5.
Let S_rad(m) be the sequence { k : rad(k) | rad(m) }. This sequence gives the number of k <= rad(m). Seen another way, this sequence gives the position of m in S_rad(m).
The number m appears after its factors in S_rad(m). If k < sqrt(m) then k^2 also appears before m.
LINKS
Michael De Vlieger, Table of n, a(n) for n = 1..1809 (all terms a(n) <= A002110(10))
Michael De Vlieger, Log log scatterplot of a(n), n = 1..1809, accentuating primorial A025487(n) in red, noting a(n) above in black, and labeling n in blue italic below for the first 24 terms.
EXAMPLE
a(1) = 1 since 1 is the only number k that does not exceed 1 such that rad(k) | 1.
a(2) = 2 since k in {1, 2} are such that rad(k) | 2.
a(3) = 3 since k in {1, 2, 4} are such that rad(k) | 4.
a(4) = 5 since k in {1, 2, 3, 4, 6} are such that rad(k) | 6, etc.
MATHEMATICA
rad[x_] := Times @@ FactorInteger[#][[All, 1]];
Map[Function[{n, r},
Count[Range[n], _?(Divisible[r, rad[#]] &)]] @@ {#, rad[#]} &,
{1}~Join~Select[Range[Times @@ Prime@ Range[6]],
# == Transpose@ {Prime@ Range[Length[#]], ReverseSort[#[[All, -1]] ]} &@
FactorInteger[#] &] ]
CROSSREFS
KEYWORD
nonn
AUTHOR
Michael De Vlieger, Oct 24 2023
STATUS
approved