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
Examples
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.
Links
- Harvey P. Dale, Table of n, a(n) for n = 1..1000
Programs
-
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 *) -
PARI
is(n)=bigomega(n)==3 && bigomega(n+2)==3 \\ Charles R Greathouse IV, Jan 31 2017
Extensions
More terms from R. J. Mathar, Aug 13 2010
Comments