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Numbers n such that string 3,1 occurs in the base 7 representation of n but not of n-1.
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%I #7 Feb 19 2024 18:01:58

%S 22,71,120,154,169,218,267,316,365,414,463,497,512,561,610,659,708,

%T 757,806,840,855,904,953,1002,1051,1078,1149,1183,1198,1247,1296,1345,

%U 1394,1443,1492,1526,1541,1590,1639,1688,1737,1786

%N Numbers n such that string 3,1 occurs in the base 7 representation of n but not of n-1.

%t SequencePosition[Table[If[SequenceCount[IntegerDigits[n,7],{3,1}]>0,1,0],{n,2000}],{0,1}][[;;,2]] (* _Harvey P. Dale_, Feb 19 2024 *)

%K nonn,base

%O 1,1

%A _Clark Kimberling_