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 A152018 #13 Oct 09 2023 11:17:44
%S A152018 9,9,81,18,225,81,441,72,729,225,1089,162,1521,441,2025,288,2601,729,
%T A152018 3249,450,3969,1089,4761,648,5625,1521,6561,882,7569,2025,8649,1152,
%U A152018 9801,2601,11025,1458,12321,3249,13689,1800,15129,3969,16641,2178,18225
%N A152018 Denominator of 1/n^2-1/(3n)^2 or of 8/(9n^2).
%C A152018 The associated terms of the n-th main series of the Hydrogen energy spectrum are A000290(3), A061038(6), A061040(9), A061042(12), A061044(15), A061046(18), A061048(21), A061050(24), etc.
%C A152018 All numbers are multiples of 9.
%H A152018 <a href="/index/Rec#order_12">Index entries for linear recurrences with constant coefficients</a>, signature (0, 0, 0, 3, 0, 0, 0, -3, 0, 0, 0, 1).
%F A152018 Sum_{n>=1} 1/a(n) = Pi^2/27 (A291050). - _Amiram Eldar_, Sep 14 2022
%t A152018 Denominator/@(8/(9Range[50]^2)) (* _Harvey P. Dale_, Mar 15 2011 *)
%Y A152018 Cf. A143025 with a similar principle of construction.
%Y A152018 Cf. A291050.
%K A152018 nonn
%O A152018 1,1
%A A152018 _Paul Curtz_, Nov 20 2008
%E A152018 Stratified definition, corrected indices, extended, _R. J. Mathar_, Dec 10 2008