OFFSET
1,2
COMMENTS
Numbers m such that if m = Product_i [p_i^e_i] then m = Product_i [e_i * (p_i^(e_i - 1))]. Example: 21600 = 2^5 * 3^3 * 5^2 = 5*2^4 * 3*3^2 * 2*5^1. - Jaroslav Krizek, Jun 23 2011
From Rémy Sigrist, Oct 29 2017: (Start)
If gcd(a(i), a(j)) = 1, then a(i)*a(j) belongs to the sequence.
This sequence has similarities with A109297, where the prime exponents are a permutation of the prime indices.
(End)
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..1000
EXAMPLE
2^5 * 3^3 * 5^2 = 21600, so 21600 is in the sequence. - corrected by Jaroslav Krizek, Jun 23 2011
MATHEMATICA
Clear[f, seq]; f[sub_] := f[sub] = (Times @@ (sub^#) & ) /@ Permutations[sub]; seq[0] = {1}; seq[k_] := seq[k] = Union[seq[k - 1], f /@ Subsets[Prime /@ Range[17], {k}] // Flatten // Union // Select[#, # <= 6836638277409177600000 &] &]; seq[k = 1]; While[nterms = Length[seq[k]]; nterms < 1000, k++; Print["nterms = ", nterms]]; seq[k] (* Jean-François Alcover, Dec 09 2013, using Alois P. Heinz's data *)
PROG
(PARI) is(n)=n=factor(n); vecsort(n[, 1])==vecsort(n[, 2]) \\ Charles R Greathouse IV, Jun 24 2011
CROSSREFS
KEYWORD
nonn
AUTHOR
Franklin T. Adams-Watters, Sep 01 2006
STATUS
approved