OFFSET
0,2
COMMENTS
Numbers n such that 2^k - (2*n+1) and (2*n+1)*2^k - 1 are both prime:
for k = 11: 6, 8, 12, ...
for k = 12: 9, ...
for k = 13: 5, 8, 10, ...
LINKS
Giovanni Resta, Table of n, a(n) for n = 0..1000
EXAMPLE
a(1) = 3 because 2^3 - (2*1+1) = 5 and (2*1+1)*2^3 - 1 = 23 are both prime, 3 = 2*1+1,
a(2) = 4 because 2^4 - (2*2+1) = 11 and (2*2+1)*2^4 - 1 = 79 are both prime, 4 < 2*2+1 = 5.
MATHEMATICA
a[n_] := Catch@ Block[{k = 1}, While[k <= 2*n+1, If[2^k - (2*n + 1) > 0 && PrimeQ[2^k - (2*n+1)] && PrimeQ[(2*n + 1)*2^k-1], Throw@k]; k++]; 0]; a/@ Range[0, 80] (* Giovanni Resta, Mar 15 2014 *)
CROSSREFS
KEYWORD
nonn
AUTHOR
Ilya Lopatin and Juri-Stepan Gerasimov, Mar 05 2014
EXTENSIONS
a(0), a(19), a(20) corrected by Giovanni Resta, Mar 13 2014
STATUS
approved