OFFSET
1,3
COMMENTS
Does a(n) exist for every n? This does not seem to be known, even on the GRH; see Heath-Brown. [Charles R Greathouse IV, Dec 27 2011]
LINKS
Pierre CAMI, Table of n, a(n) for n = 1..5000
D. R. Heath-Brown, Zero-free regions for Dirichlet L-functions, and the least prime in an arithmetic progression. Proceedings of the London Mathematical Society 64:3 (1992), pp. 265-338.
MATHEMATICA
Table[a = 0; While[! PrimeQ[(2^n - a)*2^n - 1], a++]; a, {n, 100}] (* T. D. Noe, Jun 11 2011 *)
PROG
(PARI) a(n)=forstep(k=4^n-1, 1, -2^n, if(ispseudoprime(k), return(2^n-(k+1)>>n))) \\ Charles R Greathouse IV, Dec 27 2011
CROSSREFS
KEYWORD
nonn
AUTHOR
Pierre CAMI, Jun 09 2011
STATUS
approved