OFFSET
1,1
COMMENTS
Omega(a(n) - 1) = Omega(a(n) + 1) = 3, where Omega(n)=A001222(n). In general twin k-almost prime pairs are defined by Omega(a(n) - 1) = Omega(a(n) + 1) = k. Twin 1-almost primes are twin prime pairs (A014574). - Redjan Shabani, Jul 20 2012
LINKS
Charles R Greathouse IV, Table of n, a(n) for n = 1..10000
EXAMPLE
19 is a term: 19-1 = 18 = 2*3*3 and 19+1 = 20 = 2*2*5.
MAPLE
with(numtheory); a := proc (n) if bigomega(n-1) = 3 and bigomega(n+1) = 3 then n else end if end proc: seq(a(n), n = 2 .. 1100); # Emeric Deutsch, Mar 03 2009
MATHEMATICA
q=3; lst={}; Do[If[Plus@@Last/@FactorInteger[n-1]==q&&Plus@@Last/@FactorInteger[n+1]==q, AppendTo[lst, n]], {n, 7!}]; lst
Mean/@SequencePosition[PrimeOmega[Range[1000]], {3, _, 3}] (* Requires Mathematica version 10 or later *) (* Harvey P. Dale, Mar 21 2020 *)
PROG
(PARI) is(n)=bigomega(n-1)==3 && bigomega(n+1)==3 \\ Charles R Greathouse IV, Feb 05 2017
CROSSREFS
KEYWORD
nonn
AUTHOR
Vladimir Joseph Stephan Orlovsky, Mar 01 2009
EXTENSIONS
More terms from Emeric Deutsch, Mar 03 2009
STATUS
approved