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Numbers n such that string 4,2 occurs in the base 10 representation of n but not of n-1.
0

%I #7 Mar 30 2018 10:59:40

%S 42,142,242,342,420,442,542,642,742,842,942,1042,1142,1242,1342,1420,

%T 1442,1542,1642,1742,1842,1942,2042,2142,2242,2342,2420,2442,2542,

%U 2642,2742,2842,2942,3042,3142,3242,3342,3420,3442

%N Numbers n such that string 4,2 occurs in the base 10 representation of n but not of n-1.

%t SequencePosition[Table[If[SequenceCount[IntegerDigits[n],{4,2}]>0,1,0],{n,3500}],{0,1}][[All,2]] (* Requires Mathematica version 10 or later *) (* _Harvey P. Dale_, Mar 30 2018 *)

%K nonn,base

%O 1,1

%A _Clark Kimberling_