OFFSET
1,1
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
(3*5*7 + 1)/2 = 53.
MAPLE
filter:= proc(n) local s;
if not isprime(n) then return false fi;
s:= ifactors(2*n-1)[2];
nops(s)=3 and map(t -> t[2], s)=[1, 1, 1]
end proc:
select(filter, [seq(i, i=3..1619, 2)]); # Robert Israel, May 11 2020
MATHEMATICA
t = Select[Range[1, 10000, 2], Map[Last, FactorInteger[#]] == Table[1, {3}] &]; Take[(t + 1)/2, 120] (* A234102 *)
v = Flatten[Position[PrimeQ[(t + 1)/2], True]] ; w = Table[t[[v[[n]]]], {n, 1, Length[v]}] (* A234103 *)
(w + 1)/2 (* A234104 *) (* Peter J. C. Moses, Dec 23 2013 *)
Module[{nn=100}, Select[(Times@@#+1)/2&/@Subsets[Prime[Range[nn]], {3}], PrimeQ[ #] && #<=5*Prime[nn]&]]//Union (* Harvey P. Dale, Jan 29 2023 *)
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Clark Kimberling, Dec 27 2013
STATUS
approved