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%I #7 Mar 24 2020 15:04:08
%S 1,21,37822859361,441,5842587018385982521381124421,
%T 68122318582951682301,794280046581,85766121,9261,
%U 10523016528610349434285830688358359761,1136272165922724266740722458520501,122694327386105632949003612841
%N Smallest power of 21 that begins with n.
%t With[{s=21^Range[0,100]},Table[SelectFirst[s,Take[IntegerDigits[ #], IntegerLength[ n]] == IntegerDigits[ n]&],{n,30}]]//Quiet (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Mar 24 2020 *)
%Y Cf. A180725, A180728, A180729
%K base,nonn
%O 1,2
%A Daniel Mondot (dmondot(AT)gmail.com), Sep 18 2010