OFFSET
1,1
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..1000
EXAMPLE
a(1) = 27 because (2*13+1)=(3^3) = 27.
a(2) = 50 because (7*7+1)=(2*5^2) = 50.
a(3) = 52 because (3*17+1)=(2^2*13) = 52.
a(4) = 63 because (2*31+1)=(3^2*7) = 63.
a(5) = 66 because (5*13+1)=(2*3*11) = 66.
a(6) = 70 because (3*23+1)=(2*5*7) = 70.
a(7) = 75 because (2*37+1)=(3*5^2) = 75.
a(8) = 78 because (7*11+1)=(2*3*13) = 78.
MATHEMATICA
f[n_] := Plus @@ Last /@ FactorInteger[n]; Select[Range[600], f[ # ] == 3 && f[ # - 1] == 2 &] (* Ray Chandler, Mar 20 2007 *)
Select[Select[Range[600], PrimeOmega[#]==2&]+1, PrimeOmega[#]==3&] (* Harvey P. Dale, Nov 24 2013 *)
PROG
(PARI) list(lim)=my(v=List(), t); forprime(p=2, lim, forprime(q=2, min(p, lim\p), if(bigomega(t=p*q+1)==3, listput(v, t)))); Set(v) \\ Charles R Greathouse IV, Feb 01 2017
CROSSREFS
Primes of the form semiprime + 1 are in A005385 (safe primes).
Semiprimes of the form semiprime + 1 are in A109373.
3-almost primes of the form semiprime + 1 are in this sequence.
4-almost primes of the form semiprime + 1 are in A109287.
5-almost primes of the form semiprime + 1 are in A109383.
Least n-almost prime of the form semiprime + 1 are in A128665.
KEYWORD
easy,nonn
AUTHOR
Jonathan Vos Post, Aug 24 2005
EXTENSIONS
Edited and extended by Ray Chandler, Mar 20 2007
STATUS
approved