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 A060423 #37 Feb 18 2025 17:39:03
%S A060423 0,0,0,0,0,5,6,21,24,54,60,110,120,195,210,315,336,476,504,684,720,
%T A060423 945,990,1265,1320,1650,1716,2106,2184,2639,2730,3255,3360,3960,4080,
%U A060423 4760,4896,5661,5814,6669,6840,7790,7980,9030,9240,10395,10626
%N A060423 Number of obtuse triangles made from vertices of a regular n-gon.
%H A060423 Michael De Vlieger, <a href="/A060423/b060423.txt">Table of n, a(n) for n = 0..10000</a>
%H A060423 Léo Ducas, <a href="https://pub.math.leidenuniv.nl/~vonkjb/students/2025_bachelorseminarium/kissing.pdf">Kissing Number of Craig's Lattice and Spherical Decoding</a>, Bachelor's seminar AGM Spring 2025, Leiden Univ. (Netherlands, 2024). See p. 2.
%H A060423 <a href="/index/Rec#order_07">Index entries for linear recurrences with constant coefficients</a>, signature (1,3,-3,-3,3,1,-1).
%F A060423 a(n) = n*(n-1)*(n-3)/8 when n odd; n*(n-2)*(n-4)/8 when n even.
%F A060423 G.f.: x^5*(x+5)/((1-x)(1-x^2)^3). - _Michael Somos_, Jan 30 2004
%F A060423 For n odd, a(n) = A080838(n). - _Gerald McGarvey_, Sep 14 2008
%F A060423 a(n) = n*(2*n-3-(-1)^n)*(2*n-7-(-1)^n)/32. - _Wesley Ivan Hurt_, Dec 31 2013
%F A060423 E.g.f.: x*((x - 3)*x*cosh(x) + (x^2 - x + 3)*sinh(x))/8. - _Stefano Spezia_, May 28 2022
%F A060423 a(n) = a(n-1) + 3*a(n-2) - 3*a(n-3) - 3*a(n-4) + 3*a(n-5) + a(n-6) - a(n-7). - _Mike Sheppard_, Feb 17 2025
%p A060423 A060423:=n->n*(2*n-3-(-1)^n)*(2*n-7-(-1)^n)/32; seq(A060423(n), n=0..100); # _Wesley Ivan Hurt_, Dec 31 2013
%t A060423 Table[n(2n-3-(-1)^n)(2n-7-(-1)^n)/32, {n, 0, 100}] (* _Wesley Ivan Hurt_, Dec 31 2013 *)
%t A060423 Table[If[EvenQ[n],(n(n-2)(n-4))/8,(n(n-1)(n-3))/8],{n,0,50}] (* _Harvey P. Dale_, Sep 18 2018 *)
%t A060423 LinearRecurrence[{1, 3, -3, -3, 3, 1, -1}, {0, 0, 0, 0, 0, 5, 6}, 51] (* _Mike Sheppard_, Feb 17 2025 *)
%o A060423 (PARI) a(n)=polcoeff(x^5*(5+x)/(1-x)/(1-x^2)^3+x*O(x^n),n)
%o A060423 (Magma) [n*(2*n-3-(-1)^n)*(2*n-7-(-1)^n)/32 : n in [0..60]]; // _Wesley Ivan Hurt_, Apr 14 2017
%Y A060423 Cf. A000330, A007290, A046092, A080838.
%K A060423 easy,nice,nonn
%O A060423 0,6
%A A060423 _Sen-Peng Eu_, Apr 05 2001