OFFSET
1,1
COMMENTS
Primes p such that 5*p+2, 25*p+12, 125*p+62, 625*p+312 and 3125*p+1562 are also primes. - Vincenzo Librandi, Aug 05 2010
Numbers k such that A280720(k) > 4. - Felix Fröhlich, Jan 07 2017
LINKS
John Cerkan, Table of n, a(n) for n = 1..10000
FORMULA
a(n) == 31 (mod 42). - John Cerkan, Oct 17 2016
MATHEMATICA
p5Q[n_]:=And@@PrimeQ/@NestList[5#+2&, n, 5]
Select[Prime[Range[550000]], p5Q] (* Harvey P. Dale, Feb 17 2011 *)
PROG
(Magma) [n: n in [1..10000000] | IsPrime(n) and IsPrime(5*n+2) and IsPrime(25*n+12) and IsPrime(125*n+62) and IsPrime(625*n+312) and IsPrime(3125*n+1562)] // Vincenzo Librandi, Aug 05 2010
CROSSREFS
KEYWORD
nonn
AUTHOR
STATUS
approved