OFFSET
1,1
LINKS
Vincenzo Librandi, Table of n, a(n) for n = 1..112
FORMULA
a(n) = (A143206(n) + 1)/2.
EXAMPLE
a(1) = (3* 7 + 1)/2 = 11;
a(2) = (7*11 + 1)/2 = 39;
a(3) = (13*17 + 1)/2 = 111.
MATHEMATICA
(fQ[n_]:=Block[{fi=FactorInteger@n}, Last@#&/@fi=={1, 1}&&Differences[First@#&/@fi]=={4}]; Select[Range@8000000, fQ] + 1)/2 (* Vincenzo Librandi Jan 02 2018 *)
Join[{11}, (1+Times@@#)/2&/@Select[Partition[Prime[Range[250]], 2, 1], #[[2]]- #[[1]]==4&]] (* Harvey P. Dale, Apr 04 2022 *)
PROG
(Magma) [(p*(p+4)+1)/2: p in PrimesUpTo(1000)| IsPrime(p+4)]; // Vincenzo Librandi, May 17 2014
CROSSREFS
KEYWORD
nonn
AUTHOR
Vincenzo Librandi, Aug 02 2009
EXTENSIONS
Entries checked by R. J. Mathar, Aug 07 2009
STATUS
approved