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%I #7 Jul 03 2017 11:39:31
%S 21,70,119,153,168,217,266,315,364,413,462,496,511,560,609,658,707,
%T 756,805,839,854,903,952,1001,1077,1099,1148,1182,1197,1246,1295,1344,
%U 1393,1442,1491,1525,1540,1589,1638,1687,1736,1785
%N Numbers n such that string 3,0 occurs in the base 7 representation of n but not of n+1.
%t SequencePosition[Table[If[SequenceCount[IntegerDigits[n,7],{3,0}]>0,1,0],{n,2000}],{1,0}][[All,1]] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Jul 03 2017 *)
%K nonn,base
%O 1,1
%A _Clark Kimberling_