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%I #17 Apr 08 2023 19:01:33
%S 1,2,32,4,512,64,70368744177664,8,9007199254740992,1024,
%T 1125899906842624,128,131072,140737488355328,151115727451828646838272,
%U 16,17179869184,18014398509481984,19342813113834066795298816,2048,2147483648
%N Smallest power of 2 that begins with n.
%D A. M. Yaglom and I. M. Yaglom, Challenging Mathematical Problems With Elementary Solutions, Vol. 1 pp. 29, 199-200; Prob. 91a, Dover NY 1987
%H Christian N. K. Anderson, <a href="/A018802/b018802.txt">Table of n, a(n) for n = 1..981</a> (a(982) has 1196 digits)
%F a(n) = 2^A018856(n). - _Seiichi Manyama_, Jan 29 2017
%F a(2^n) = 2^n for n >= 0. - _Seiichi Manyama_, Jan 29 2017
%e a(7) = 70368744177664, because 2^46 is the smallest power of 2 that begins with a 7.
%o (R) library(gmp); ndig<-function(i) nchar(as.character(as.bigz(i))) y=as.bigz(rep(0,100)); for(i in 1:100) {n=as.bigz(2); while(substr(n,1,ndig(i))!=as.character(i)) n=n*2; y[i]=n; } # _Christian N. K. Anderson_, May 23 2013
%Y Cf. A018856.
%Y Cf. this sequence (k=2), A018857 (k=3), A018859 (k=4), A018861 (k=5), A018863 (k=6), A018865 (k=7), A018867 (k=8), A018869 (k=9).
%K nonn,base
%O 1,2
%A _David W. Wilson_
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