OFFSET
1,1
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..1000
EXAMPLE
15 = 3*5 is the least product of distinct primes p and q for which (p*q - 1)/2 is prime, so a(1) = 15.
MATHEMATICA
t = Select[Range[1, 7000, 2], Map[Last, FactorInteger[#]] == Table[1, {2}] &]; Take[(t - 1)/2, 120] (* A234093 *)
v = Flatten[Position[PrimeQ[(t - 1)/2], True]] ; w = Table[t[[v[[n]]]], {n, 1, Length[v]}] (* A233561 *)
(w - 1)/2 (* A234095 *) (* Peter J. C. Moses, Dec 23 2013 *)
With[{upto=2000}, Select[Times@@#&/@Select[Subsets[Prime[Range[ PrimePi[ upto/2]]], {2}], PrimeQ[(Times@@#-1)/2]&]//Union, #<=upto&]] (* Harvey P. Dale, Nov 02 2017 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Clark Kimberling, Dec 14 2013
STATUS
approved