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 A305624 Number of chiral pairs of rows of n colors with exactly 4 different colors. 2
 0, 0, 0, 12, 120, 780, 4188, 20400, 93120, 409140, 1748220, 7337232, 30386160, 124696740, 508250988, 2061566400, 8331954240, 33585590580, 135115594140, 542784981552, 2178107091600, 8733341736900, 34996103558988, 140172672276000, 561255446475360, 2246716252964820, 8991948337723260, 35983044114659472, 143977928423467440, 576048972752188260, 2304607666801990188, 9219666007300387200, 36882370043723748480 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,4 COMMENTS If the row is achiral, i.e., the same as its reverse, we ignore it. If different from its reverse, we count it and its reverse as a chiral pair. LINKS FORMULA a(n) = (k!/2) * (S2(n,k) - S2(ceiling(n/2),k)), with k=4 colors used and where S2(n,k) is the Stirling subset number A008277. a(n) = (A000919(n) - A056455(n)) / 2. a(n) = A000919(n) - A056311(n) = A056311(n) - A056455(n). G.f.: -(k!/2) * (x^(2k-1) + x^(2k)) / Product_{j=1..k} (1 - j*x^2) + (k!/2) * x^k / Product_{j=1..k} (1 - j*x) with k=4 colors used. EXAMPLE For a(4) = 12, the chiral pairs are the 4! = 24 permutations of ABCD, each paired with its reverse. MATHEMATICA k=4; Table[(k!/2) (StirlingS2[n, k] - StirlingS2[Ceiling[n/2], k]), {n, 1, 40}] PROG (PARI) a(n) = my(k=4); (k!/2) * (stirling(n, k, 2) - stirling(ceil(n/2), k, 2)); \\ Michel Marcus, Jun 07 2018 CROSSREFS Fourth column of A305622. A056455(n) is number of achiral rows of n colors with exactly 4 different colors. Cf. A000919, A056311. Sequence in context: A093334 A001816 A133386 * A056320 A056311 A009050 Adjacent sequences:  A305621 A305622 A305623 * A305625 A305626 A305627 KEYWORD nonn,easy AUTHOR Robert A. Russell, Jun 06 2018 STATUS approved

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Last modified August 20 05:25 EDT 2019. Contains 326139 sequences. (Running on oeis4.)