A180117 Numbers n such that n and n+2 are both divisible by exactly 3 primes (counted with multiplicity).

18, 28, 42, 50, 66, 68, 76, 114, 170, 172, 186, 188, 236, 242, 244, 266, 273, 282, 284, 290, 316, 343, 354, 385, 402, 404, 410, 423, 426, 428, 434, 436, 475, 506, 596, 602, 603, 604, 637, 652, 663, 668, 722, 762, 775, 786, 788, 845, 890, 892, 906, 925, 962

Offset

1,1

Comments

Numbers k such that both k and k + 2 are in A014612.

Links

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

Example

a(1) = 18 because 18 = 2*3*3 and 18+2 = 20 = 2*2*5 both have 3 prime divisors, counted with multiplicity.
a(2) = 28 because 28 = 2*2*7 and 28+2 = 30 = 2*3*5 both have 3 prime divisors, counted with multiplicity.

Mathematica

#[[1, 1]]&/@(Select[Partition[Table[{n, PrimeOmega[n]}, {n, 1000}], 3, 1], #[[1, 2]]==#[[3, 2]]==3&]) (* Harvey P. Dale, Oct 20 2011 *)
SequencePosition[PrimeOmega[Range[1000]], {3, _, 3}][[All, 1]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Apr 08 2017 *)

Prog

(PARI) is(n)=bigomega(n)==3 && bigomega(n+2)==3 \\ Charles R Greathouse IV, Jan 31 2017

Crossrefs

Cf. A001359, A014612.

Sequence in context: A216259 A117101 A063840 * A339633 A167333 A259642

Adjacent sequences: A180114 A180115 A180116 * A180118 A180119 A180120

Keyword

easy,nonn

Author

Jonathan Vos Post, Aug 10 2010

Extensions

More terms from R. J. Mathar, Aug 13 2010

Status

approved