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 A240530 #27 Sep 08 2022 08:46:07
%S A240530 4,8,24,80,280,1008,3696,13728,51480,194480,739024,2821728,10816624,
%T A240530 41602400,160466400,620470080,2404321560,9334424880,36300541200,
%U A240530 141381055200,551386115280,2153031497760,8416395854880,32933722910400,128990414732400
%N A240530 a(n) = 4*(2*n)! / (n!)^2.
%C A240530 Apart from first term, the same as A146534. - _Arkadiusz Wesolowski_, Apr 12 2014
%H A240530 Vincenzo Librandi, <a href="/A240530/b240530.txt">Table of n, a(n) for n = 0..1000</a>
%H A240530 M. Pedrazzi and G. Goldoni, <a href="http://www.academia.edu/436065/Un_labirinto_cartesiano_A_Cartesian_Labyrinth_">Un labirinto cartesiano (A Cartesian Labyrinth)</a>, Archimede, Anno XXXVIII, Jan-Mar 1986, p. 41 (in Italian).
%F A240530 G.f.: 4/sqrt(1-4*x).
%F A240530 a(n) = 4*binomial(2*n, n) = 4*A000984(n) = 2*A028329(n).
%F A240530 D-finite with recurrence: n*a(n) - 2*(2*n-1)*a(n-1) = 0 for n > 0.
%p A240530 seq( 4*binomial(2*n,n), n=0..30); # _G. C. Greubel_, Dec 19 2019
%t A240530 Table[4*(2*n)!/(n!)^2, {n, 0, 40}] (* or *) CoefficientList[Series[4/Sqrt[1 - 4 x], {x, 0, 50}], x]
%o A240530 (Magma) [4*Binomial (2*n,n): n in [0..30]];
%o A240530 (PARI) vector(31, n, 4*binomial(2*n-2, n-1)) \\ _G. C. Greubel_, Dec 19 2019
%o A240530 (Sage) [4*binomial(2*n,n) for n in (0..30)] # _G. C. Greubel_, Dec 19 2019
%o A240530 (GAP) List([0..30], n-> 4*Binomial(2*n,n) ); # _G. C. Greubel_, Dec 19 2019
%Y A240530 Cf. A000984, A028329, A146534.
%K A240530 nonn
%O A240530 0,1
%A A240530 _Vincenzo Librandi_, Apr 12 2014