OFFSET
1,4
COMMENTS
If a(n)=1, then the two primes are same and they are Mersenne primes (A000668).
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..1000
EXAMPLE
a(9) = 25 because 2^9 - 25 = 487 and 25*2^9 - 1 = 12799 are both prime.
MAPLE
a:= proc(n) local k, p; p:= 2^n;
for k while not (isprime(p-k) and isprime(k*p-1))
do if k>=p then return 0 fi od; k
end:
seq(a(n), n=1..70); # Alois P. Heinz, Mar 03 2014
MATHEMATICA
a[n_] := Module[{k, p}, p = 2^n;
For[k = 1, !(PrimeQ[p - k] && PrimeQ[k*p - 1]), k++,
If[k >= p, Return[0]]]; k];
Table[a[n], {n, 1, 70}] (* Jean-François Alcover, Feb 18 2022, after Alois P. Heinz *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Lopatin and Juri-Stepan Gerasimov, Mar 03 2014
EXTENSIONS
More terms from Alois P. Heinz, Mar 03 2014
STATUS
approved