A258373 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.

1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 102, 202, 203, 303, 304, 404, 405, 505, 506, 606, 607, 707, 708, 808, 809, 909, 910, 20, 103, 302, 204, 403, 305, 504, 406, 605, 507, 706, 608, 807, 709, 908, 810, 19, 92, 30, 104, 402, 205, 503, 306, 604, 407, 705, 508, 806, 609, 907, 710, 18

Offset: 1

Author

Eric Angelini and M. F. Hasler, Jul 23 2015

Keywords

• nonn
• base
• easy

Examples

			The concatenation of the first two terms is 12, and
12 - 1 = 11 is a palindrome, and similar for all subsequent terms:
23 - 1 = 22,
34 - 1 = 33,
...
910 - 1 = 909,
10102 - 1 = 10101,
...
9230 - 1 = 9229 is a palindrome, ...
		

Links

• M. F. Hasler, Table of n, a(n) for n = 1..1816
• E. Angelini, A palindrome + 1, SeqFan list, July 21, 2015.

Programs

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))}