Search: id:A048639
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%I A048639
%S A048639 3,5,9,6,10,17,33,18,65,12,129,34,257,66,20,130,513,1025,36,258,2049,
%T A048639 24,4097,68,8193,514,40,1026,16385,132,32769,2050,260,65537,72,131073,
%U A048639 4098,8194,136,262145,16386,524289,48,516,1048577,1028,2097153,32770
%N A048639 Binary encoding of A006881, numbers with two distinct prime divisors.
%F A048639 a(n) = 2^(i-1)+2^(j-1), where A006881[ n ] = p_i*p_j (p_i and p_j stand 
               for the i-th and j-th primes respectively, where the first prime 
               is 2).
%p A048639 encode_A006881 := proc(upto_n) local b,i; b := [ ]; for i from 1 to upto_n 
               do if((0 <> mobius(i)) and (4 = tau(i))) then b := [ op(b), bef(i) 
               ]; fi; od: RETURN(b); end; # see A048623 for bef
%Y A048639 Permutation of A018900. Cf. A048640, A048623.
%Y A048639 Sequence in context: A101298 A138055 A076844 this_sequence A021282 A016613 
               A079427
%Y A048639 Adjacent sequences: A048636 A048637 A048638 this_sequence A048640 A048641 
               A048642
%K A048639 nonn
%O A048639 1,1
%A A048639 Antti Karttunen, Jul 14 1999

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