A114974 Numbers n such that 7^n + n^7 is a semiprime.

2, 12, 16, 18, 30, 39, 160, 214, 235, 408

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

Links

Table of n, a(n) for n=1..10.

Status of 7^510-510^7 in factordb.com.

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

Cf. A114970, A114971, A114973.

Sequence in context: A120300 A089021 A176888 * A298001 A071566 A295821

Adjacent sequences: A114971 A114972 A114973 * A114975 A114976 A114977

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