A336339 Numbers composite(n) such that gcd(n,composite(n)) is even.

6, 12, 18, 24, 26, 28, 32, 34, 36, 42, 48, 50, 52, 60, 68, 70, 74, 76, 78, 84, 86, 88, 98, 100, 104, 106, 110, 112, 128, 130, 138, 150, 158, 160, 162, 168, 170, 172, 180, 192, 198, 212, 214, 216, 218, 220, 222, 228, 234, 236, 238, 242, 244, 246, 248, 250

Offset

1,1

Links

Harvey P. Dale, Table of n, a(n) for n = 1..1000

Example

In the following table, c(n) = A002808(n) = composite(n).
  n     c(n)   gcd(n, c(n))
  1      4         1
  2      6         2
  3      8         1
  4      9         1
  5      10        5
  6      12        6
2 and 6 are in A336338; 6 and 12 are in A336339; 1 and 3 are in A336340; 3 and 6 are in A336341.

Mathematica

c = Select[Range[2, 200], ! PrimeQ[#] &];  (*  A002808 *)
u = Select[Range[Length[c]], EvenQ[GCD[c[[#]], #]] &]   (* A336338 *)
v = Table[c[[u[[n]]]], {n, 1, Length[u]}]; (* A336339 *)
u/2  (* A336340 *)
v/2  (* A336341 *)
With[{nn=300}, {cmps=Select[Range[nn], CompositeQ]}, Select[Thread[{cmps, Range[Length[cmps]]}], EvenQ[GCD@@#]&]][[;; , 1]] (* Harvey P. Dale, Jan 16 2026 *)

Crossrefs

Cf. A002808, A336338, A336340, A336341.

Sequence in context: A037981 A044846 A037917 * A282146 A204879 A326696

Adjacent sequences: A336336 A336337 A336338 * A336340 A336341 A336342

Keyword

nonn,easy

Author

Clark Kimberling, Oct 03 2020

Status

approved