%I A057821
%S A057821 1,5,9,5,21,29,9,5,9,17,45,161,165,269,285,17,45,233,9,17,321,317,
%T A057821 633,677,405,437,189,1385,69,209,9,641,849,137,45,401,381,437,1965,
%U A057821 2201,741,1493,573,857,1485,5297,2709,161,465,473,1269,4805,789
%N A057821 Useful safe primes: a(n) = least nonnegative integer k such that 2^n 
               - k is prime and (2^n-k-1)/2 is also prime.
%Y A057821 Cf. A013603.
%Y A057821 Sequence in context: A117014 A010720 A094389 this_sequence A133742 A134879 
               A051158
%Y A057821 Adjacent sequences: A057818 A057819 A057820 this_sequence A057822 A057823 
               A057824
%K A057821 nonn
%O A057821 3,2
%A A057821 Warren Smith (wds(AT)research.nj.nec.com), Nov 23 2000