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%I #32 Jan 02 2023 12:30:51
%S 1,2,3,4,5,6,7,8,9,10,102,202,203,303,304,404,405,505,506,606,607,707,
%T 708,808,809,909,910,20,103,302,204,403,305,504,406,605,507,706,608,
%U 807,709,908,810,19,92,30,104,402,205,503,306,604,407,705,508,806,609,907,710,18
%N a(n+1) is the least number not occurring earlier such that concatenation(a(n),a(n+1)) - 1 is a palindrome; a(1)=1.
%H M. F. Hasler, <a href="/A258373/b258373.txt">Table of n, a(n) for n = 1..1816</a>
%H E. Angelini, <a href="http://list.seqfan.eu/oldermail/seqfan/2015-July/015099.html">A palindrome + 1</a>, SeqFan list, July 21, 2015.
%e The concatenation of the first two terms is 12, and
%e 12 - 1 = 11 is a palindrome, and similar for all subsequent terms:
%e 23 - 1 = 22,
%e 34 - 1 = 33,
%e ...
%e 910 - 1 = 909,
%e 10102 - 1 = 10101,
%e ...
%e 9230 - 1 = 9229 is a palindrome, ...
%o (PARI) {S=[a=1];for(i=1,100,print1(a",");for(k=1,9e9,Vecrev(d=digits(eval(Str(a,k))-1))==d||next;setsearch(S,k)&&next;S=setunion(S,[a=k]);break))}
%K nonn,base,easy
%O 1,2
%A _Eric Angelini_ and _M. F. Hasler_, Jul 23 2015
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