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%I #3 Mar 30 2012 17:28:26
%S 50,51,25,145,142,119,86,239,93,1558,1598,761,1183,6651,5515,4183,34,
%T 8990,1343,4180,7987,10628,938,5519,4177,10213,10326,6652,1327,5516,
%U 12326,4184,11964,6928,6649,8991,653,10373,11549,4181,3706,7988,5862,10629,8988,939,13084
%N The initial decimal digits of 2^a(n) are the decimal digits of n followed by n.
%D E.I. Ignatjew, Mathematische Spielereien, Urania Verlag Leipzig-Jena-Berlin, 2. Auflage 1982
%e a(1) = 50; 2^50 = 1125899906842624
%e a(2) = 51; 2^51 = 2251799813685248
%e a(3) = 25; 2^25 = 33554432
%e a(10) = 1558; 2^1558 = 10109583053...744
%o (PARI) double(n,k)=n\=10^(#Str(n)-2*k);if(n%10^k==n\=10^k,n)
%o a(n)=my(s=#Str(n));for(k=1,1e6,if(double(1<<k,s)==n,return(k)))
%Y Cf. A018802, A171132, A171242, A171489, A171617
%K nonn,base
%O 1,1
%A Eva-Maria Zschorn (e-m.zschorn(AT)zaschendorf.km3.de), Dec 14 2009
%E More terms, program, and editing by _Charles R Greathouse IV_, Aug 02 2010