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%I #7 May 21 2021 17:33:43
%S 11,36,59,61,86,111,136,161,184,186,211,236,261,299,309,311,336,361,
%T 386,411,434,436,461,486,511,536,559,561,586,611,636,661,684,686,711,
%U 736,761,786,809,811,836,861,886,924,934,936,961
%N Numbers n such that string 2,1 occurs in the base 5 representation of n but not of n+1.
%t SequencePosition[Table[If[SequenceCount[IntegerDigits[n,5],{2,1}]>0,1,0],{n,1000}],{1,0}][[All,1]] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, May 21 2021 *)
%K nonn,base
%O 1,1
%A _Clark Kimberling_
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