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Milan Janjic, <a href="https://pmf.unibl.org/wp-content/uploads/2017/10/enumfun.pdf">Enumerative Formulas for Some Functions on Finite Sets</a>.
0, 0, 24, 72, 144, 240, 360, 504, 672, 864, 1080, 1320, 1584, 1872, 2184, 2520, 2880, 3264, 3672, 4104, 4560, 5040, 5544, 6072, 6624, 7200, 7800, 8424, 9072, 9744, 10440, 11160, 11904, 12672, 13464, 14280, 15120, 15984, 16872, 17784, 18720, 19680, 20664, 21672
Sum_{n>=12} 1/a(n) = 1/12.
Sum_{n>=12} (-1)^(n+1)/a(n) = (2*log(2) - 1)/12.
Product_{n>=12} (1 - 1/a(n)) = -(12/Pi)*cos(Pi/sqrt(3)).
Product_{n>=12} (1 + 1/a(n)) = (12/Pi)*cos(Pi/sqrt(6)). (End)
L. Luigi Berzolari, Allgemeine Theorie der Höheren Ebenen Algebraischen Kurven, Encyclopädie der Mathematischen Wissenschaften mit Einschluss ihrer Anwendungen. , Band III_2. , Heft 3, Leipzig: B. G. Teubner, 1906. , p. 341.
Milan Janjic, <a href="httphttps://wwwpmf.pmfblunibl.org/janjicwp-content/uploads/2017/10/enumfun.pdf">Enumerative Formulas for Some Functions on Finite Sets</a>
Leo Tavares, <a href="/A064200/a064200.jpg">Illustration: Twin Stars</a>.
From Amiram Eldar, Feb 22 2023: (Start)
Sum_{n>=1} 1/a(n) = 1/12.
Sum_{n>=1} (-1)^(n+1)/a(n) = (2*log(2) - 1)/12.
Product_{n>=1} (1 - 1/a(n)) = -(12/Pi)*cos(Pi/sqrt(3)).
Product_{n>=1} (1 + 1/a(n)) = (12/Pi)*cos(Pi/sqrt(6)). (End)
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