OFFSET
1,1
COMMENTS
455 is not a term of the sequence, since 455 = 5*7*13 is sphenic, i.e., the number of distinct prime factors is 3, though 456 = 2^3*3*19 has 3 distinct prime factors but is not sphenic, because the number of prime factors with repetition is 5 > 3.
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..1000
MAPLE
Sphenics:= select(t -> (map(s->s[2], ifactors(t)[2])=[1, 1, 1]), {$1..10000}):
Sphenics intersect map(`-`, Sphenics, 1); # Robert Israel, Aug 13 2014
MATHEMATICA
Select[Range[2500], (PrimeNu[#] == PrimeOmega[#] == PrimeNu[#+1] == PrimeOmega[#+1] == 3)&] (* Jean-François Alcover, Apr 11 2014 *)
SequencePosition[Table[If[PrimeNu[n]==PrimeOmega[n]==3, 1, 0], {n, 2500}], {1, 1}][[All, 1]] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Aug 02 2017 *)
PROG
(Haskell)
twinLow [] = []
twinLow [_] = []
twinLow (n : (m : ns))
| m == n + 1 = n : twinLow (m : ns)
| otherwise = twinLow (m : ns)
a215217 n = (twinLow a007304_list) !! (n - 1)
-- Peter Dolland, May 31 2019
(PARI) is_a033992(n) = omega(n)==3 && bigomega(n)==3
is(n) = is_a033992(n) && is_a033992(n+1) \\ Felix Fröhlich, Jun 10 2019
CROSSREFS
KEYWORD
nonn
AUTHOR
Martin Renner, Aug 06 2012
STATUS
approved