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 A175188 #20 Aug 14 2023 06:28:30
%S A175188 201,40001,500001,6000001,70000001,800000001,100000000001,
%T A175188 1100000000001,12000000000001,130000000000001,1400000000000001,
%U A175188 15000000000000001,160000000000000001,1700000000000000001,18000000000000000001,190000000000000000001,2000000000000000000001
%N A175188 Composite numbers of the form k*10^k + 1.
%D A175188 M. Abramowitz and I. A. Stegun, eds., Handbook of Mathematical Functions, National Bureau of Standards Applied Math. Series 55, 1964 (and various reprintings), p. 844.
%H A175188 M. Abramowitz and I. A. Stegun, eds., <a href="http://www.convertit.com/Go/ConvertIt/Reference/AMS55.ASP">Handbook of Mathematical Functions</a>, National Bureau of Standards, Applied Math. Series 55, Tenth Printing, 1972 [alternative scanned copy].
%e A175188 For k=2, 2*10^2 + 1 = 201 = 3*67.
%e A175188 For k=4, 4*10^4 + 1 = 40001 = 13*17*181.
%p A175188 with(numtheory):for k from 1 to 100 do: n:=k*10^k +1:if type(n,prime) = false then print(n):else fi:od:
%t A175188 Select[Table[k*10^k+1, {k, 1, 20}], CompositeQ] (* _Amiram Eldar_, Aug 14 2023 *)
%Y A175188 Intersection of A002808 and A064748.
%Y A175188 Subsequence of A005381.
%Y A175188 Cf. A216347.
%K A175188 nonn
%O A175188 1,1
%A A175188 _Michel Lagneau_, Mar 01 2010
%E A175188 a(16)-a(17) added and name corrected by _Amiram Eldar_, Aug 14 2023