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 A335600 #13 Jun 27 2020 16:59:55 %S A335600 2,1,110,10,1101,11010,3,330,30,3303,33030,4,440,40,4404,44040,5,550, %T A335600 50,5505,55050,6,660,60,6606,66060,7,770,70,7707,77070,8,880,80,8808, %U A335600 88080,9,990,90,9909,99090,11,101,1010,22,20,202,220,2022,2020,33,303,3030,44,404,4040,55,505,5050,66,606,6060,77 %N A335600 The poor sandwiches sequence (see Comments lines for definition). %C A335600 Imagine we would have a pair of adjacent integers in the sequence like [1951, 2020]. The sandwich would then be made of the rightmost digit of a(n), the leftmost digit of a(n+1) and, in between, the absolute difference of those two digits. The pair [1951, 2020] would then produce the (poor) sandwich 112. (Why poor? Because a rich sandwich would insert the sum of the digits instead of their absolute difference - that is 132 in this example). Please note that the pair [2020, 1951] would produce the poor and genuine sandwich 011 (we keep the leading zero: these are sandwiches after all, not integers). %C A335600 Now we want the sequence to be the lexicographically earliest sequence of distinct positive terms such that the successive sandwiches emerging from the sequence rebuild it, digit after digit. %H A335600 Carole Dubois, <a href="/A335600/b335600.txt">Table of n, a(n) for n = 1..125</a> %e A335600 The first successive sandwiches are: 211, 101, 011, 011, 101, 033,... %e A335600 The first one (211) is visible between a(1) = 2 and a(2) = 1; we get the sandwich by inserting the difference 1 between 2 and 1. %e A335600 The second sandwich (101) is visible between a(2) = 1 and a(3) = 110; we get this sandwich by inserting the difference 0 between 1 and 1. %e A335600 The third sandwich (011) is visible between a(3) = 110 and a(4) = 10; we get this sandwich by inserting the difference 1 between 0 and 1; etc. %e A335600 The successive sandwiches rebuild, digit by digit, the starting sequence. %Y A335600 Cf. A086005 (Semiprimes sandwiched between semiprimes). %K A335600 base,nonn %O A335600 1,1 %A A335600 _Eric Angelini_ and _Carole Dubois_, Jun 15 2020