OFFSET
1,1
COMMENTS
627 and 748 are sequence terms < 1100. a(11) >= 510. Unknown factorization also for 622 and 790. - Hugo Pfoertner, Jul 28 2019
EXAMPLE
2 is in the sequence because 7^2 + 2^7 = 177 = 3*59 (semiprime).
12 is in the sequence because 7^12 + 12^7 = 13*1067470693 (semiprime). [Vincenzo Librandi, Dec 16 2010]
PROG
(Magma)IsSemiprime:=func<n | &+[k[2]: k in Factorization(n)] eq 2>; [n: n in [1..85] | IsSemiprime(7^n+n^7)] // Vincenzo Librandi, Dec 16 2010
CROSSREFS
KEYWORD
nonn,more,hard
AUTHOR
Zak Seidov, Feb 22 2006
EXTENSIONS
Corrected (inserted 12) from Vincenzo Librandi, Dec 16 2010
a(7) from D. S. McNeil, Dec 16 2010
a(8)-a(10) from Luke March, Aug 03 2015
STATUS
approved