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%I #8 Nov 21 2013 12:47:03
%S 21,121,210,221,321,421,521,621,721,821,921,1021,1121,1210,1221,1321,
%T 1421,1521,1621,1721,1821,1921,2021,2100,2210,2221,2321,2421,2521,
%U 2621,2721,2821,2921,3021,3121,3210,3221,3321,3421
%N Numbers n such that string 2,1 occurs in the base 10 representation of n but not of n-1.
%t Select[Range[3500],MemberQ[Partition[IntegerDigits[#],2,1],{2,1}] && !MemberQ[Partition[IntegerDigits[#-1],2,1],{2,1}]&] (* _Harvey P. Dale_, Sep 24 2011 *)
%K nonn,base
%O 1,1
%A _Clark Kimberling_
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