OFFSET
1,1
COMMENTS
Numbers k such that k-1 and k^6+k^5+k^4+k^3+k^2+k+1 are both prime.
LINKS
Robert Israel, Table of n, a(n) for n = 1..10000
EXAMPLE
3 is in sequence because 3^7-1 = 2186 = 2*1093, where 2 and 1093 are both prime.
MAPLE
with(numtheory): A261436:=n->`if`(bigomega(n^7-1)=2, n, NULL): seq(A261436(n), n=1..2000); # Wesley Ivan Hurt, Aug 21 2015
select(n -> isprime(n-1) and isprime(n^6+n^5+n^4+n^3+n^2+n+1), [3, (2*i $i=2..10000)]); # Robert Israel, Aug 21 2015
MATHEMATICA
Select[Range[5000], PrimeOmega[#^7 - 1] == 2 &]
PROG
(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [2..5000] | IsSemiprime(s) where s is n^7- 1];
(PARI) isok(n)=bigomega(n^7-1)==2 \\ Anders Hellström, Aug 21 2015
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Vincenzo Librandi, Aug 21 2015
STATUS
approved