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 A374624 #25 Dec 08 2024 01:38:38
%S A374624 1,-1,3,5,3,4,4,4,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,
%T A374624 3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,
%U A374624 3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3,3
%N A374624 a(n) is the number of irreducible finite Coxeter groups in n dimensions, or -1 if there are an infinite number.
%C A374624 For n > 8, the Coxeter groups are exactly A(n), B(n) = C(n), and D(n), hence a(n) = 3.
%D A374624 H. S. M. Coxeter, Regular Polytopes, Dover Publications, Inc., 1973.
%H A374624 <a href="/index/Rec#order_01">Index entries for linear recurrences with constant coefficients</a>, signature (1).
%F A374624 G.f.: (1 - 2*x + 4*x^2 + 2*x^3 - 2*x^4 + x^5 - x^8)/(1 - x). - _Stefano Spezia_, Jul 15 2024
%e A374624 For n = 4, there are five finite groups, denoted A(4) (symmetry group of the simplex), B(4) (= C(4)) (symmetry group of the tesseract and the 4-dimensional cross polytope), D(4) (symmetry group of the demitesseract), F(4) (symmetry group of the 24-cell) and H(4) (symmetry group of the 120-cell and the 600-cell).
%t A374624 PadRight[{1, -1, 3, 5, 3, 4, 4, 4}, 100, 3] (* _Paolo Xausa_, Dec 07 2024 *)
%o A374624 (PARI) a(n)=if(n>8,3,[1,-1,3,5,3,4,4,4][n]) \\ _Charles R Greathouse IV_, Jul 15 2024
%Y A374624 Cf. A060296, A358241.
%K A374624 easy,sign
%O A374624 1,3
%A A374624 _Douglas Boffey_, Jul 14 2024