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 A055426 #21 Aug 03 2025 09:03:24
%S A055426 1,5,13,33,89,221,485,953,1713,2869,4541,6865,9993,14093,19349,25961,
%T A055426 34145,44133,56173,70529,87481,107325,130373,156953,187409,222101,
%U A055426 261405,305713,355433,410989,472821,541385,617153,700613,792269,892641
%N A055426 Number of points in Z^n of norm <= 2.
%H A055426 Andrew Howroyd, <a href="/A055426/b055426.txt">Table of n, a(n) for n = 0..1000</a>
%H A055426 <a href="/index/Rec#order_05">Index entries for linear recurrences with constant coefficients</a>, signature (5,-10,10,-5,1).
%F A055426 a(n) = (3+2*n+16*n^2-8*n^3+2*n^4)/3. - corrected by _Colin Barker_, Jul 07 2013
%F A055426 G.f.: -(x+1)*(9*x^3-x^2-x+1) / (x-1)^5. - _Colin Barker_, Jul 07 2013
%F A055426 a(0)=1, a(1)=5, a(2)=13, a(3)=33, a(4)=89, a(n)=5*a(n-1)-10*a(n-2)+ 10*a(n-3)- 5*a(n-4)+a(n-5). - _Harvey P. Dale_, Feb 09 2015
%t A055426 LinearRecurrence[{5,-10,10,-5,1},{1,5,13,33,89},40] (* _Harvey P. Dale_, Feb 09 2015 *)
%Y A055426 Row n=2 of A302997. Column 4 of A122510.
%K A055426 nonn,easy
%O A055426 0,2
%A A055426 _David W. Wilson_