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Numbers n such that string 1,2 occurs in the base 3 representation of n but not of n-1.
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%I #7 Jan 28 2020 14:08:55

%S 5,14,23,32,41,59,68,77,86,95,104,113,122,167,176,185,194,203,221,230,

%T 239,248,257,266,275,284,302,311,320,329,338,347,356,365,491,500,509,

%U 518,527,545,554,563,572,581,590,599,608,653

%N Numbers n such that string 1,2 occurs in the base 3 representation of n but not of n-1.

%t SequencePosition[Table[If[SequenceCount[IntegerDigits[n,3],{1,2}]>0,1,0],{n,700}],{0,1}][[All,2]] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Jan 28 2020 *)

%K nonn,base

%O 1,1

%A _Clark Kimberling_