%I A156660
%S A156660 0,0,1,1,0,1,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,1,0,0,0,0,0,
%T A156660 0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0,
%U A156660 0,0,0,0,0,0,0,0,0,0,0,0,0,1,0,0,0,0,0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0,0
%N A156660 Characteristic function of Sophie Germain primes.
%C A156660 a(n) = if n and also 2*n+1 is prime then 1 else 0; a(A005384(n)) = 1; 
               a(A138887(n)) = 0; a(A053176(n)) = 0.
%C A156660 A156874(n) = SUM(a(k): 1<=k<=n). [From Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), 
               Feb 18 2009]
%H A156660 R. Zumkeller, <a href="b156660.txt">Table of n, a(n) for n = 1..10000</
               a>
%H A156660 <a href="Sindx_Ch.html#char_fns">Index entries for characteristic functions</
               a>
%H A156660 Wikipedia, <a href="http://en.wikipedia.org/wiki/Sophie_Germain_prime">
               Sophie Germain prime</a>
%F A156660 a(n) = A010051(n)*A010051(2*n+1).
%Y A156660 A156659.
%Y A156660 Sequence in context: A091225 A132380 A021913 this_sequence A155899 A117814 
               A062301
%Y A156660 Adjacent sequences: A156657 A156658 A156659 this_sequence A156661 A156662 
               A156663
%K A156660 nonn
%O A156660 0,1
%A A156660 Reinhard Zumkeller (reinhard.zumkeller(AT)gmail.com), Feb 13 2009
%E A156660 Definition corrected by Daniel Forgues (squid(AT)zensearch.com), Aug 
               04 2009