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 A037487 #35 Jan 01 2024 11:04:37
%S A037487 1,12,121,1212,12121,121212,1212121,12121212,121212121,1212121212,
%T A037487 12121212121,121212121212,1212121212121,12121212121212,
%U A037487 121212121212121,1212121212121212,12121212121212121,121212121212121212,1212121212121212121,12121212121212121212
%N A037487 Decimal expansion of a(n) is given by the first n terms of the periodic sequence with initial period 1,2.
%C A037487 See A037610 for a general formula. - _Hieronymus Fischer_, Jan 03 2013
%C A037487 (Smoothly undulating palindromic) primes in this sequence are listed in A092696(n) = (4*10^A062209(n)-7)/33. - _M. F. Hasler_, Jul 30 2015
%H A037487 Hieronymus Fischer, <a href="/A037487/b037487.txt">Table of n, a(n) for n = 1..200</a>
%H A037487 <a href="/index/Rec">Index entries for linear recurrences with constant coefficients</a>, signature (10,1,-10).
%F A037487 a(n) = floor((4/33)*10^n). - _Hieronymus Fischer_, Jan 03 2013
%F A037487 a(n) = 10*a(n-1)+a(n-2)-10*a(n-3). G.f.: x*(2*x+1) / ((x-1)*(x+1)*(10*x-1)). - _Colin Barker_, Apr 30 2014
%t A037487 Table[FromDigits[PadRight[{},n,{1,2}]],{n,20}] (* or *) LinearRecurrence[ {10,1,-10},{1,12,121},20] (* _Harvey P. Dale_, Jun 21 2016 *)
%o A037487 (PARI) A037487(n)=10^n*4\33 \\ - _M. F. Hasler_, Jan 13 2013
%o A037487 (PARI) Vec(x*(2*x+1)/((x-1)*(x+1)*(10*x-1)) + O(x^100)) \\ _Colin Barker_, Apr 30 2014
%Y A037487 Cf. A037610.
%K A037487 nonn,base
%O A037487 1,2
%A A037487 _Clark Kimberling_