OFFSET
1,1
COMMENTS
Primes p such that 5*p+2, 25*p+12, 125*p+62 and 625*p+312 are also primes. - Vincenzo Librandi, Aug 04 2010
Numbers k such that A280720(k) > 3. - Felix Fröhlich, Jan 07 2017
LINKS
John Cerkan, Table of n, a(n) for n = 1..10000
FORMULA
a(n) == 31 or 37 (mod 42). - John Cerkan, Oct 07 2016
MATHEMATICA
Select[Range[10^6], Times @@ Boole@ Map[PrimeQ, NestList[5 # + 2 &, #, 4]] == 1 &] (* Michael De Vlieger, Jan 09 2017 *)
PROG
(Magma) [n: n in [1..1000000] | IsPrime(n) and IsPrime(5*n+2) and IsPrime(25*n+12) and IsPrime(125*n+62) and IsPrime(625*n+312)]; // Vincenzo Librandi, Aug 04 2010
CROSSREFS
KEYWORD
nonn
AUTHOR
STATUS
approved