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 A070252 #21 Jul 31 2025 22:03:29
%S A070252 10,9,90,90,900,900,9000,9000,90000,90000,900000,900000,9000000,
%T A070252 9000000,90000000,90000000,900000000,900000000,9000000000,9000000000,
%U A070252 90000000000,90000000000,900000000000,900000000000,9000000000000
%N A070252 Number of n-digit palindromes.
%H A070252 <a href="/index/Rec#order_02">Index entries for linear recurrences with constant coefficients</a>, signature (0,10).
%F A070252 From _Colin Barker_, Aug 19 2013: (Start)
%F A070252 a(n) = 10*a(n-2) for n>3.
%F A070252 G.f.: x*(10*x^2-9*x-10) / (10*x^2-1). (End)
%F A070252 E.g.f.: x + 9*(cosh(sqrt(10)*x) - 1 + sqrt(10)*sinh(sqrt(10)*x))/10. - _Stefano Spezia_, Jun 11 2022
%t A070252 LinearRecurrence[{0,10},{10,9,90},25] (* _Stefano Spezia_, Jun 11 2022 *)
%o A070252 (Python)
%o A070252 def A070252(n): return 10 if n==1 else 9*10**(n-1>>1) # _Chai Wah Wu_, Jul 30 2025
%Y A070252 A variant of A050683, which is the principal entry for this sequence. Cf. A016115.
%Y A070252 Partial sums give A070199.
%K A070252 base,nonn,easy
%O A070252 1,1
%A A070252 _Amarnath Murthy_, May 06 2002