A163489 - OEIS

A163489

Indices n such that composite(n)/3 is prime.

2, 4, 8, 12, 21, 26, 35, 40, 49, 63, 68, 81, 92, 97, 106, 121, 136, 140, 154, 165, 171, 185, 195, 210, 229, 240, 245, 254, 260, 270, 305, 315, 330, 336, 360, 365, 379, 395, 405, 421, 437, 442, 467, 472, 483, 488, 517, 547, 557, 562, 573, 589, 594, 619, 634, 650

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OFFSET

1,1

LINKS

Table of n, a(n) for n=1..56.

EXAMPLE

a(1)=2 (2nd composite/3 = 6/3 = 2, which is prime).

a(2)=4 (4th composite/3 = 9/3 = 3, which is prime).

MATHEMATICA

Position[Select[Range[1000], CompositeQ], _?(PrimeQ[#/3]&)]//Flatten (* Harvey P. Dale, Aug 06 2022 *)

CROSSREFS

Cf. A000040, A002808.