A229630 a(n) is the smallest semiprime m such that 2*m^k-1 is prime for k = 1, 2, ..., n.
4, 4, 4, 6, 6, 118909855, 5740959589, 79235997091
Offset: 1
Examples
a(5)=6 because 2*6^k-1 is prime for k=1,2,3,4,5 and 6 is the smallest semiprime with this property. Also 6 is the smallest such number.
Programs
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Mathematica
a[n_]:=(For[m=1,!(2
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PARI
\\ Code to find a(8), can be modified to find other terms easily issemi(n)=bigomega(n)==2 is8(m)=for(i=2,8,if(!ispseudoprime(2*m^i-1),return(0)));1 forprime(p=2,1e12,m=(p+1)/2;if(issemi(m)&&is8(m),return(m))) \\ Charles R Greathouse IV, Oct 17 2013
Extensions
a(8) from Charles R Greathouse IV, Oct 17 2013