A354446 11-gonal numbers (numbers of the form k*(9*k-7)/2), which are products of three distinct primes.

30, 506, 606, 715, 1558, 1730, 3945, 5083, 6365, 8558, 9361, 11986, 12455, 14935, 15458, 17081, 19371, 19966, 21183, 25726, 29971, 32215, 32981, 37766, 45551, 46461, 51146, 54065, 57065, 58083, 62245, 68758, 74433, 75595, 76766, 80333, 86458, 88971, 90241

Offset

1,1

Comments

A squarefree subsequence of 11-gonal numbers.

Links

Table of n, a(n) for n=1..39.

Example

     30 =   3*(9*3   - 7)/2 =  2 *  3 *   5;
    506 =  11*(9*11  - 7)/2 =  2 * 11 *  23;
   3945 =  30*(9*30  - 7)/2 =  3 *  5 * 263;
  80333 = 134*(9*134 - 7)/2 = 11 * 67 * 109.

Maple

q:= n-> is(map(x-> x[2], ifactors(n)[2])=[1$3]):
select(q, [n*(9*n-7)/2$n=1..200])[];  # Alois P. Heinz, Jun 15 2022

Mathematica

Select[Table[n*(9*n-7)/2, {n, 1, 150}], FactorInteger[#][[;; , 2]]=={1, 1, 1} &] (* Amiram Eldar, Jun 01 2022 *)

Crossrefs

Intersection of A051682 and A007304.

Sequence in context: A090957 A060560 A118681 * A333648 A020926 A229563

Adjacent sequences: A354443 A354444 A354445 * A354447 A354448 A354449

Keyword

nonn

Author

Massimo Kofler, Jun 01 2022

Status

approved