OFFSET
1,1
COMMENTS
After 2, numbers k such that k-1 and k^10 + k^9 + k^8 + k^7 + k^6 + k^5 + k^4 + k^3 + k^2 + k + 1 are both prime.
EXAMPLE
20 is in sequence because 20^11-1 = 204799999999999 = 19*10778947368421, where 19 and 10778947368421 are both prime.
MATHEMATICA
Select[Range[6000], PrimeOmega[#^11 - 1] == 2 &]
PROG
(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [2..4000] | IsSemiprime(s) where s is n^11- 1];
(PARI) isok(n)=bigomega(n^11-1)==2 \\ Anders Hellström, Aug 21 2015
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Vincenzo Librandi, Aug 21 2015
STATUS
approved