login
The OEIS is supported by the many generous donors to the OEIS Foundation.

 

Logo
Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A334880 Numbers k such that gcd(k, k-th composite number) > 1. 2
2, 5, 6, 7, 10, 12, 14, 15, 16, 18, 20, 21, 22, 24, 26, 28, 30, 32, 34, 36, 42, 48, 50, 51, 52, 54, 55, 56, 60, 62, 63, 64, 65, 70, 72, 74, 75, 76, 77, 78, 80, 81, 82, 93, 96, 98, 99, 102, 104, 105, 110, 112, 114, 120, 122, 124, 126, 128, 130, 132, 138, 148 (list; graph; refs; listen; history; text; internal format)
OFFSET
1,1
LINKS
EXAMPLE
In the following table, c(k) = A002808(k) = k-th composite number.
k c(k) gcd(k, c(k))
1 4 1
2 6 2
3 8 1
4 9 1
5 10 5
6 12 6
2, 5, 6 are in this sequence, and 1,3,4 are in A073258.
MATHEMATICA
c = Select[Range[2, 150], ! PrimeQ[#] &]; (* A002808 *)
Select[Range[Length[c]], GCD[c[[#]], #] > 1 &] (* A334880 *)
Module[{nn=200, cmps, len}, cmps=Select[Range[nn], CompositeQ]; len=Length[ cmps]; Select[Thread[{Range[len], cmps}], GCD@@#>1&]][[All, 1]] (* Harvey P. Dale, Sep 20 2020 *)
PROG
(PARI) lista(nn) = {my(n=0, list=List()); forcomposite (c=1, nn, n++; if (gcd(n, c) > 1, listput(list, n))); Vec(list); } \\ Michel Marcus, Jul 19 2020
CROSSREFS
Cf. A002808, A064814, A073258 (complement), A336323.
Sequence in context: A286751 A293278 A087791 * A285032 A002157 A050002
KEYWORD
nonn,easy
AUTHOR
Clark Kimberling, Jul 17 2020
STATUS
approved

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

License Agreements, Terms of Use, Privacy Policy. .

Last modified February 26 04:47 EST 2024. Contains 370335 sequences. (Running on oeis4.)