A257586 - OEIS

A257586

Numbers that are neither primes nor three times primes.

1, 4, 8, 10, 12, 14, 16, 18, 20, 22, 24, 25, 26, 27, 28, 30, 32, 34, 35, 36, 38, 40, 42, 44, 45, 46, 48, 49, 50, 52, 54, 55, 56, 58, 60, 62, 63, 64, 65, 66, 68, 70, 72, 74, 75, 76, 77, 78, 80, 81, 82, 84, 85, 86, 88, 90, 91, 92, 94, 95, 96

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OFFSET

1,2

COMMENTS

Arises in studying A254077.

LINKS

N. J. A. Sloane, Table of n, a(n) for n = 1..10000

FORMULA

a(n) ~ n*(1+(4/3)/log(n)).

MAPLE

s1:=[]: for n from 1 to 1000 do

if isprime(n) or ( (n mod 3) = 0 and isprime(n/3)) then ; else s1:=[op(s1), n]; fi; od: s1;

MATHEMATICA

Select[Range@96, Nor[PrimeQ[#], PrimeQ[#/3]]&] (* Ivan N. Ianakiev, May 08 2015 *)

Module[{nn=100, pr, pr3}, pr=Prime[Range[PrimePi[nn]]]; pr3=3Prime[Range[ PrimePi[ nn/3]]]; Complement[Range[nn], pr, pr3]] (* Harvey P. Dale, May 13 2020 *)

CROSSREFS

Cf. A254077.

Sequence in context: A155878 A029886 A102218 * A172458 A175226 A057166

Adjacent sequences: A257583 A257584 A257585 * A257587 A257588 A257589

KEYWORD

nonn

AUTHOR

N. J. A. Sloane, May 07 2015

STATUS

approved