This site is supported by donations to The OEIS Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A122406 Numbers of the form Product_i p_i^e_i, where the p_i are distinct primes and the e_i are a permutation of the p_i. 9
 1, 4, 27, 72, 108, 800, 3125, 6272, 12500, 21600, 30375, 36000, 48600, 84375, 121500, 169344, 225000, 247808, 337500, 395136, 750141, 823543, 857304, 1384448, 3000564, 3294172, 6690816, 19600000, 22235661, 24532992, 37380096, 37879808 (list; graph; refs; listen; history; text; internal format)
 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 Subsequence of A054411, A054412, and A122405. Cf. A109297. Sequence in context: A272859 A054412 A122405 * A276372 A071837 A266011 Adjacent sequences:  A122403 A122404 A122405 * A122407 A122408 A122409 KEYWORD nonn AUTHOR Franklin T. Adams-Watters, Sep 01 2006 STATUS approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified October 22 06:54 EDT 2019. Contains 328315 sequences. (Running on oeis4.)