A229627
a(n) is the smallest prime q such that 2*q^k - 1 is prime for k = 1, 2, ..., n.
Original entry on oeis.org
2, 2, 3, 92581, 385939, 464938699, 24137752519, 1095265755949
Offset: 1
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a[1]=2;a[n_]:=a[n]=(For[m=PrimePi[a[n-1]],Union[Table[PrimeQ[2 Prime[m]^k-1],{k,n}]]!={True},m++];Prime[m])
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a(n)=forprime(m=2,,for(k=1,n,if(!ispseudoprime(2*m^k-1), next(2))); return(m)) \\ Charles R Greathouse IV, Oct 01 2013
A229902
a(n) is the smallest m such that 19*m^k+1 is prime for k=1, 2,...,n.
Original entry on oeis.org
10, 10, 10, 2080, 178030, 41697270, 4355942080, 1944616927560
Offset: 1
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a[1]=10;a[n_]:=a[n]=(For[m=a[n-1]/10,Union[Table[PrimeQ[19(10m)^k+1],{k,n}]]=={True},m++];10m) (* Farideh Firoozbakht Oct 05 2013 *)
A229630
a(n) is the smallest semiprime m such that 2*m^k-1 is prime for k = 1, 2, ..., n.
Original entry on oeis.org
4, 4, 4, 6, 6, 118909855, 5740959589, 79235997091
Offset: 1
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.
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a[n_]:=(For[m=1,!(2
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\\ 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
Showing 1-3 of 3 results.
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