OFFSET
1,1
COMMENTS
The semiprimes of this form are: 705895, 46118409, 2824752491, 9495123019887, 71213422649146, ...
a(13) >= 720. - Kevin P. Thompson, Apr 20 2022
LINKS
MATHEMATICA
Select[Range[80], PrimeOmega[# 7^# + 1] == 2 &]
PROG
(Magma) IsSemiprime:=func<i | &+[d[2]: d in Factorization(i)] eq 2>; [n: n in [1..80] | IsSemiprime(s) where s is n*7^n+1];
(PARI) is(k) = bigomega(k*7^k+1)==2;
for(k=0, 120, if(k%4!=1, if(is(k), print1(k, ", ")))); \\ Jinyuan Wang, Apr 07 2019
CROSSREFS
KEYWORD
nonn,more
AUTHOR
Vincenzo Librandi, May 10 2014
EXTENSIONS
a(7)-a(9) from Jinyuan Wang, Apr 07 2019
a(10)-a(12) from Kevin P. Thompson, Apr 20 2022
STATUS
approved