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 A195042 #34 Jan 16 2023 08:18:59
%S A195042 0,1,9,19,36,55,81,109,144,181,225,271,324,379,441,505,576,649,729,
%T A195042 811,900,991,1089,1189,1296,1405,1521,1639,1764,1891,2025,2161,2304,
%U A195042 2449,2601,2755,2916,3079,3249,3421,3600,3781,3969,4159,4356,4555,4761,4969,5184,5401,5625
%N A195042 Concentric 9-gonal numbers.
%C A195042 Also concentric enneagonal numbers or concentric nonagonal numbers.
%C A195042 A016766 and A069131 interleaved.
%C A195042 Partial sums of A056020. - _Reinhard Zumkeller_, Jan 07 2012
%H A195042 Vincenzo Librandi, <a href="/A195042/b195042.txt">Table of n, a(n) for n = 0..10000</a>
%H A195042 <a href="/index/Rec#order_04">Index entries for linear recurrences with constant coefficients</a>, signature (2,0,-2,1).
%F A195042 a(n) = (9*n^2 + 5/2*((-1)^n - 1))/4.
%F A195042 From _R. J. Mathar_, Sep 28 2011: (Start)
%F A195042 G.f.: -x*(1+7*x+x^2) / ( (1+x)*(x-1)^3 ).
%F A195042 a(n) + a(n+1) = A060544(n+1). (End)
%F A195042 Sum_{n>=1} 1/a(n) = Pi^2/54 + tan(sqrt(5)*Pi/6)*Pi/(3*sqrt(5)). - _Amiram Eldar_, Jan 16 2023
%t A195042 LinearRecurrence[{2,0,-2,1},{0,1,9,19},60] (* _Harvey P. Dale_, Nov 24 2019 *)
%o A195042 (Magma) [(9*n^2+5/2*((-1)^n-1))/4: n in [0..50]]; // _Vincenzo Librandi_, Sep 29 2011
%o A195042 (Haskell)
%o A195042 a195042 n = a195042_list !! n
%o A195042 a195042_list = scanl (+) 0 a056020_list
%o A195042 -- _Reinhard Zumkeller_, Jan 07 2012
%o A195042 (PARI) a(n)=(9*n^2+5/2*((-1)^n-1))/4 \\ _Charles R Greathouse IV_, Oct 07 2015
%Y A195042 Column 9 of A195040.
%Y A195042 Cf. A016766, A069131, A077221, A195041, A195142, A195043.
%K A195042 nonn,easy
%O A195042 0,3
%A A195042 _Omar E. Pol_, Sep 27 2011