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 A056313 Number of reversible strings with n beads using exactly six different colors. 4
 0, 0, 0, 0, 0, 360, 7560, 95760, 952560, 8217720, 64615680, 476515080, 3355679880, 22837101840, 151449674040, 984573656640, 6302070915840, 39847411326600, 249509384858160, 1550188410555960, 9570844671224760 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,6 COMMENTS A string and its reverse are considered to be equivalent. REFERENCES M. R. Nester (1999). Mathematical investigations of some plant interaction designs. PhD Thesis. University of Queensland, Brisbane, Australia. [See A056391 for pdf file of Chap. 2] LINKS Index entries for linear recurrences with constant coefficients, signature (19, -117, 81, 1883, -5915, -6615, 53235, -30394, -191744, 264852, 258804, -634248, 43920, 505440, -259200). FORMULA a(n) = A056308(n) - 6*A032122(n) + 15*A032121(n) - 20*A032120(n) + 15*A005418(n+1) - 6. G.f.: 360*x^6*(8*x^2 - x - 1)*(90*x^7 - 9*x^6 - 29*x^5 - 34*x^4 + 15*x^3 + 9*x^2 - x - 1)/((x - 1)*(2*x - 1)*(2*x + 1)*(3*x - 1)*(4*x - 1)*(5*x - 1)*(6*x - 1)*(2*x^2 - 1)*(3*x^2 - 1)*(5*x^2 - 1)*(6*x^2 - 1)). - Colin Barker, Sep 03 2012 a(n) = k! (S2(n,k) + S2(ceiling(n/2),k)) / 2, where k=6 is the number of colors and S2 is the Stirling subset number. - Robert A. Russell, Sep 25 2018 EXAMPLE For n=6, the 360 rows are 360 permutations of ABCDEF that do not include any mutual reversals.  Each of the 360 chiral pairs, such as ABCDEF-FEDCBA, is then counted just once. - Robert A. Russell, Sep 25 2018 MATHEMATICA k=6; Table[(StirlingS2[i, k]+StirlingS2[Ceiling[i/2], k])k!/2, {i, k, 30}] (* Robert A. Russell, Nov 25 2017 *) PROG (PARI) a(n) = my(k=6); k!/2*(stirling(n, k, 2) + stirling(ceil(n/2), k, 2)); \\ Altug Alkan, Sep 27 2018 CROSSREFS Cf. A056308, A056322. Column 6 of A305621. Equals (A000920 + A056457) / 2 = A000920 - A305626 = A305626 + A056457. Sequence in context: A033592 A305626 A056322 * A192829 A229681 A054648 Adjacent sequences:  A056310 A056311 A056312 * A056314 A056315 A056316 KEYWORD nonn,easy AUTHOR STATUS approved

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Last modified September 26 06:05 EDT 2020. Contains 337346 sequences. (Running on oeis4.)