%I #7 Dec 06 2018 16:40:38
%S 9,58,63,107,156,205,254,303,352,401,406,441,499,548,597,646,695,744,
%T 749,793,842,891,940,989,1038,1087,1092,1136,1185,1234,1283,1332,1381,
%U 1430,1435,1479,1528,1577,1626,1675,1724,1773
%N Numbers n such that string 1,2 occurs in the base 7 representation of n but not of n-1.
%t SequencePosition[Table[If[SequenceCount[IntegerDigits[n,7],{1,2}]>0,1,0],{n,2000}],{0,1}][[All,2]] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Dec 06 2018 *)
%K nonn,base
%O 1,1
%A _Clark Kimberling_