A252995 Numbers n such that the n-th odd composite number is 3n.

335, 337, 339, 343, 349, 353, 359, 365, 367, 369, 371, 373, 375, 377

Offset

1,1

Comments

The sequence is finite, since lim A071904(n)/n = 2.

Links

Table of n, a(n) for n=1..14.

Formula

Define b(k) = 6k + 3 for k > 0. Delete all terms except those such that (b(k) - 1)/2 - pi(k) + 1 = b(k)/3. Then a(k) = b(k)/3. - Alonso del Arte, Dec 25 2014

Example

a(1) = 335, since, for n <= 334, A071904(n) > 3*n, while A071904(335) = 3*335.
a(14) = 377 is the last term, since, A071904(377) = 3*377, while A071904(n) < 3*n, for n >= 378.

Mathematica

Select[Range[9, 1197, 6], (# - 1)/2 - PrimePi[#] + 1 == #/3 &]/3 (* Alonso del Arte, Dec 25 2014 *)
Module[{nn=10000, cmps}, cmps=Select[Range[nn], CompositeQ[#]&&OddQ[#]&]; Select[Thread[ {Range[Length[cmps]], cmps}], #[[2]]==3#[[1]]&]][[;; , 1]] (* Harvey P. Dale, Nov 22 2023 *)

Crossrefs

Cf. A071904.

Sequence in context: A278098 A303209 A216138 * A119760 A046015 A184076

Adjacent sequences: A252992 A252993 A252994 * A252996 A252997 A252998

Keyword

nonn,fini,full

Author

Vladimir Shevelev, Dec 25 2014

Status

approved