A242270 Numbers k such that k*7^k+1 is semiprime.

6, 8, 10, 14, 15, 60, 90, 114, 118, 204, 350, 390

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

Table of n, a(n) for n=1..12.

FactorDB, Status of 720*7^720+1

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

Cf. similar sequences listed in A242203.

Cf. A050919, A242177.

Sequence in context: A030513 A161918 A294729 * A298252 A152126 A315853

Adjacent sequences: A242267 A242268 A242269 * A242271 A242272 A242273

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