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 A006587 #25 Aug 28 2025 00:41:51
%S A006587 3,36,720,16128,380160,9225216,228114432,5715394560,144599482368,
%T A006587 3685869158400,94513444945920,2435255516528640,62999001405849600,
%U A006587 1635260233414606848,42568679092062781440,1110895735754245275648
%N A006587 a(n) = 3*2^(2*n)*(3*n)!/((2*n)!*n!).
%D A006587 W. A. Whitworth, DCC Exercises in Choice and Chance, Stechert, NY, 1945, p. 35.
%H A006587 Delbert L. Johnson, <a href="/A006587/b006587.txt">Table of n, a(n) for n = 0..699</a>
%H A006587 M. Le Brun, <a href="/A006577/a006577.pdf">Email to N. J. A. Sloane, Jul 1991</a>
%F A006587 From _G. C. Greubel_, Aug 27 2025: (Start)
%F A006587 a(n) = 3 * A006588(n).
%F A006587 G.f.: 3*hypergeometric2F1([1/3, 2/3], [1/2], 27*x) = (3/(2*(1-27*x))*( cos(t) + cos(2*t) ), where t = (1/3)*arccos(1-54*x).
%F A006587 E.g.f.: 3*hypergeometric2F2([1/3, 2/3], [1/2, 1], 27*x). (End)
%p A006587 A006587:=n->3*2^(2*n)*(3*n)!/((2*n)!*n!); seq(A006587(n), n=0..50); # _Wesley Ivan Hurt_, Nov 23 2013
%t A006587 Table[3*2^(2n)(3n)!/((2n)!*n!), {n, 0, 50}] (* _Wesley Ivan Hurt_, Nov 23 2013 *)
%o A006587 (PARI) a(n)=3*binomial(3*n,n)*4^n \\ _Charles R Greathouse IV_, Aug 11 2017
%o A006587 (Magma)
%o A006587 A006587:= func< n | 3*4^n*Binomial(3*n,n) >;
%o A006587 [A006587(n): n in [0..40]]; // _G. C. Greubel_, Aug 27 2025
%o A006587 (SageMath)
%o A006587 def A006587(n): return 3*4**n*binomial(3*n,n)
%o A006587 print([A006587(n) for n in range(41)]) # _G. C. Greubel_, Aug 27 2025
%Y A006587 Cf. A006588.
%K A006587 nonn,changed
%O A006587 0,1
%A A006587 _N. J. A. Sloane_