This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A056142 #9 Jul 11 2020 20:03:07
%S A056142 0,1,2,3,4,5,6,7,8,9,101,111,121,131,141,151,161,171,181,191,202,212,
%T A056142 222,232,242,252,262,272,282,292,303,313,323,333,343,353,363,373,383,
%U A056142 393,404,414,424,434,444,454,464,474,484,494,505,515,525,535,545,555
%N A056142 Concatenate n, floor[n/10], floor[n/100] ... (but do not continue if floor[.]=0).
%C A056142 For 0 < n < 100, a(n) = A056525(n). If n has 3 digits, then a(n) is a palindrome if and only if n is. If n has 4 or 5 digits, then a(n) is a palindrome if and only if all digits of n are equal. - _David Wasserman_, May 23 2005
%C A056142 Conjecture: if n has 3 or more digits, a(n) is a palindrome only if all the digits of n are the same. It is easy to see that any palindrome can have at most 2 distinct digits: matching digits from the initial n in the concatenation matches each digit after the second with an earlier digit. - _Franklin T. Adams-Watters_, Sep 07 2006
%t A056142 Table[FromDigits[Flatten[IntegerDigits/@DeleteCases[Floor[n/10^Range[ 0,5]],0]]],{n,0,60}] (* _Harvey P. Dale_, Jul 11 2020 *)
%Y A056142 Cf. A002113, A056143, A056525.
%K A056142 base,easy,nonn
%O A056142 0,3
%A A056142 _Henry Bottomley_, Jun 15 2000