OFFSET
1,1
LINKS
Harvey P. Dale, Table of n, a(n) for n = 1..274 (all terms up to 20000)
EXAMPLE
97 = (3*5*13 - 1)/2, and 3*5*13 is the least product p*q*r of 3 distinct primes for which (p*q*r - 1)/2 is prime, so a(1) = 3*5*13.
MATHEMATICA
t = Select[Range[1, 10000, 2], Map[Last, FactorInteger[#]] == Table[1, {3}] &]; Take[(t - 1)/2, 120] (* A234099 *)
v = Flatten[Position[PrimeQ[(t - 1)/2], True]] ; w = Table[t[[v[[n]]]], {n, 1, Length[v]}] (* this sequence *)
(w - 1)/2 (* A234101 *) (* Peter J. C. Moses, Dec 23 2013 *)
With[{upto=4000}, Select[Union[Times@@@Select[Subsets[Prime[ Range[ PrimePi[ upto/ 6]]], {3}], PrimeQ[(Times@@#-1)/2]&]], #<=upto&]] (* Harvey P. Dale, May 12 2017 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Clark Kimberling, Dec 27 2013
STATUS
approved