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%I #12 Oct 14 2019 03:47:42
%S 6,10,12,18,22,32,49,52,53,56,78
%N Numbers k such that 2^k contains 2^2=4 as its largest proper substring of the form 2^m (probably finite).
%t sub2[n_] := Block[{s = ToString[2^n], k = n - 1}, While[k >= 0 && ! StringContainsQ[s, ToString[2^k]], k--]; k]; Select[Range[1000], sub2[#] == 2 &] (* _Giovanni Resta_, Oct 14 2019 *)
%Y Cf. A033921.
%K nonn,base,more
%O 1,1
%A _Patrick De Geest_, Jun 15 1998
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