%I #11 Aug 28 2021 02:50:59
%S 12,21,112,129,219,221,312,321,412,421,512,521,612,621,712,721,812,
%T 821,912,921,1012,1021,1112,1129,1299,1312,1321,1412,1421,1512,1521,
%U 1612,1621,1712,1721,1812,1821,1912,1921,2012,2021
%N Numbers k such that 1 and 2 occur juxtaposed in the base-10 representation of k but not of k+1.
%H Michael De Vlieger, <a href="/A044005/b044005.txt">Table of n, a(n) for n = 1..10000</a>
%t With[{j = 1, k = 2}, SequencePosition[Table[If[SequenceCount[IntegerDigits[n], {j, k}] > 0 || SequenceCount[IntegerDigits[n], {k, j}] > 0, 1, 0], {n, 2100}], {1, 0}][[All, 1]]] (* _Michael De Vlieger_, Aug 27 2021 *)
%K nonn,base
%O 1,1
%A _Clark Kimberling_
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