login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A216918 Odd numbers with at least 3 distinct prime factors. 2
105, 165, 195, 231, 255, 273, 285, 315, 345, 357, 385, 399, 429, 435, 455, 465, 483, 495, 525, 555, 561, 585, 595, 609, 615, 627, 645, 651, 663, 665, 693, 705, 715, 735, 741, 759, 765, 777, 795, 805, 819, 825, 855, 861, 885, 897, 903, 915, 935, 945, 957, 969 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,1

COMMENTS

If "at least" is changed to "exactly" we get A278569. - N. J. A. Sloane, Nov 27 2016

LINKS

Alois P. Heinz, Table of n, a(n) for n = 1..10000

J. B. Cosgrave, K. Dilcher, Extensions of the Gauss-Wilson Theorem, Integers: Electronic Journal of Combinatorial Number Theory, 8 (2008), p.11.

FORMULA

Gauss_factorial(floor(a(n)/2), a(n)) == 1 (mod a(n)). (Cf. A216919)

MAPLE

a:= proc(n) option remember; local k;

      for k from 2+ `if`(n=1, 103, a(n-1)) by 2

        while nops(numtheory[factorset](k))<=2 do od; k

    end:

seq (a(n), n=1..100);  # Alois P. Heinz, Oct 03 2012

MATHEMATICA

Select[Range[1, 999, 2], (PrimeNu[#] >= 3)&] (* Jean-François Alcover, Feb 27 2014 *)

PROG

(Sage)

def is_A216918(n):

    if n%2 == 0: return false

    if len(n.prime_divisors()) >= 3: return true

    return false

def A216918_list(n): return filter(is_A216918, (1..n))

A216918_list(969)

CROSSREFS

A278569 is a subsequence.

Sequence in context: A252069 A133509 A013590 * A278569 A046389 A154430

Adjacent sequences:  A216915 A216916 A216917 * A216919 A216920 A216921

KEYWORD

nonn

AUTHOR

Peter Luschny, Oct 02 2012

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.

License Agreements, Terms of Use, Privacy Policy. .

Last modified February 22 10:29 EST 2019. Contains 320395 sequences. (Running on oeis4.)