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 A138898 #18 Aug 19 2025 05:25:50
%S A138898 60,840,30240,1995840,207567360,31135104000,6351561216000,
%T A138898 1689515283456000,567677135241216000,235018333989863424000,
%U A138898 117509166994931712000000,69800445194989436928000000,48581109855712648101888000000,39156374543704394370121728000000,36180490078382860397992476672000000,37989514582302003417892100505600000000,44979585265445572046784246998630400000000,59642930061980828534035911520183910400000000
%N A138898 Ratio of (2*n-1)! to number of zeros in lower part of Sylvester matrix for polynomial of degree n with all nonzero coefficients.
%F A138898 a(n) = (2*n-1)!/((n-1)*(n-2)). - _R. J. Mathar_, Apr 30 2008
%F A138898 Sum_{n=3..oo} 1/a(n) = (3*cosh(1) + 10*sinh(1) - 6*e)/4 = 0.0178907175323686230239526350278045532... . - _Stefano Spezia_, Jul 27 2024
%F A138898 Equivalently, Sum_{n=3..oo} 1/a(n) = (e^2 - 7)/(8*e). - _Vaclav Kotesovec_, Aug 19 2025
%F A138898 Sum_{n>=3} (-1)^(n+1)/a(n) = (2*sin(1) - 3*cos(1))/4. - _Amiram Eldar_, Aug 19 2025
%t A138898 Table[(2 n - 1)!/((n - 1)(n - 2)), {n, 3, 20}] (* _R. J. Mathar_, Apr 30 2008 *)
%Y A138898 Cf. A002378, A009445, A007878, A138896.
%K A138898 nonn,easy
%O A138898 3,1
%A A138898 _Artur Jasinski_, Apr 02 2008
%E A138898 Edited and corrected by _R. J. Mathar_, Apr 30 2008