Search: id:A126646
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%I A126646
%S A126646 3,7,15,31,63,127,255,511,1023,2047,4095,8191,16383,32767,65535,131071,
%T A126646 262143,524287,1048575,2097151,4194303,8388607,16777215,33554431,
%U A126646 67108863,134217727,268435455,536870911,1073741823,2147483647
%N A126646 a(n) is the number of integers k less than 10^n such that the decimal 
               representation of k lacks the digits 1,2,3,4,5,6 and 7 and at least 
               one of digits 8,9.
%C A126646 Numbers n such that [a(n-1)]^2+a(n) is square [From Vincenzo Librandi 
               (vincenzo.librandi(AT)tin.it), Dec 07 2009]
%H A126646 Milan Janjic, <a href="http://www.pmfbl.org/janjic/">Enumerative Formulas 
               for Some Functions on Finite Sets</a>
%F A126646 a(n)=2*2^n-1
%e A126646 a(8)=511
%p A126646 f:=n->2*2^n-1;
%Y A126646 Cf. A125630, A125948, A125947, A125946, A125945, A125940, A125909, A125908, 
               A125880, A125897, A125904, A125858.
%Y A126646 Sequence in context: A097002 A060152 A168604 this_sequence A000225 A123121 
               A117060
%Y A126646 Adjacent sequences: A126643 A126644 A126645 this_sequence A126647 A126648 
               A126649
%K A126646 nonn,new
%O A126646 1,1
%A A126646 Aleksandar M. Janjic and Milan R. Janjic (agnus(AT)blic.net), Feb 08 
               2007, Feb 13 2007

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