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

Author

Vincenzo Librandi, May 10 2014

Keywords

• nonn
• more

Comments

The semiprimes of this form are: 705895, 46118409, 2824752491, 9495123019887, 71213422649146, ...

a(13) >= 720. - Kevin P. Thompson, Apr 20 2022

Links

• FactorDB, Status of 720*7^720+1

Crossrefs

Cf. similar sequences listed in A242203.

Cf. A050919, A242177.

Programs

Magma

IsSemiprime:=func; [n: n in [1..80] | IsSemiprime(s) where s is n*7^n+1];

Mathematica

Select[Range[80], PrimeOmega[# 7^# + 1] == 2 &]

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

Extensions

a(7)-a(9) from Jinyuan Wang, Apr 07 2019

a(10)-a(12) from Kevin P. Thompson, Apr 20 2022