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 A193431 #16 Nov 16 2021 16:53:24
%S A193431 123,234,345,456,567,678,789,891,910,101,11,111,112,121,213,131,314,
%T A193431 141,415,151,516,161,617,171,718,181,819,192,920,202,21,212,122,222,
%U A193431 223,232,324,242,425,252,526,262,627,272,728,282,829,293,930,303,31,313
%N A193431 Put the natural numbers together without spaces and read them three at a time advancing one space each time.
%H A193431 Reinhard Zumkeller, <a href="/A193431/b193431.txt">Table of n, a(n) for n = 1..10000</a>
%F A193431 a(n) = 10 * (10 * A007376(n) + A007376(n+1)) + A007376(n+2).
%t A193431 Module[{nn=50,d},d=Flatten[IntegerDigits/@Range[nn]];FromDigits/@ Partition[ d,3,1]] (* _Harvey P. Dale_, Nov 16 2021 *)
%o A193431 (Haskell)
%o A193431 a193431 n = a193431_list !! (n-1)
%o A193431 a193431_list = f a007376_list where
%o A193431 f (x:xs'@(x':x'':xs)) = 10*(10*x + x') + x'' : f xs'
%Y A193431 Cf. A136414, A193492, A193493.
%K A193431 nonn,base
%O A193431 1,1
%A A193431 _Reinhard Zumkeller_, Jul 28 2011