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 A006204 Number of starters in cyclic group of order 2n+1. (Formerly M2802) 1
 1, 1, 3, 9, 25, 133, 631, 3857, 25905, 188181, 1515283, 13376125, 128102625, 1317606101, 14534145947, 170922533545, 2138089212789 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,3 COMMENTS A complete mapping of a cyclic group (Z_m,+) is a permutation f(x) of Z_m with f(0)=0 such that f(x)-x is also a permutation. a(n) is the number of complete mappings f(x) of the cyclic group Z_{2n+1} such that f^(-1)=f. In other words a(n) is the number of complete mappings fixed under the reflection operator R, where R(f)=f^(-1). Reflection R is not only a symmetry operator of complete mappings, but also one of the (Toroidal)-(semi) N-Queen problems and of the strong complete mappings problem. REFERENCES CRC Handbook of Combinatorial Designs, 1996, p. 469. CRC Handbook of Combinatorial Designs, 2nd edition, 2007, p. 624. J. D. Horton, Orthogonal starters in finite Abelian groups, Discrete Math., 79 (1989/1990), 265-278. V. Linja-aho and P. R. J. Ostergard, Classification of starters, J. Combin. Math. Combin. Comput. 75 (2010), 153-159. Y. P. Shieh, "Partition strategies for #P-complete problems with applications to enumerative combinatorics", PhD thesis, National Taiwan University, 2001. Y. P. Shieh, J. Hsiang and D. F. Hsu, "On the enumeration of Abelian k-complete mappings", vol. 144 of Congressus Numerantium, 2000, pp. 67-88. N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence). LINKS V. Linja-aho and P. R. J. Ostergard, Classification of starters, J. Combin. Math. Combin. Comput. 75 (2010), 153-159. Y. P. Shieh, Cyclic complete mappings counting problems EXAMPLE f(x)=6x in (Z_7,+) is a complete mapping of Z_7 since that f(0)=0 and that f(x)-x (=5x) is also a permutation of Z_7. f^(-1)(x)=6x=f(x). So f(x) is fixed under reflection. CROSSREFS Cf. A006717, A071607, A071608, A071706, A003111. Sequence in context: A178061 A120284 A074440 * A013572 A119851 A119825 Adjacent sequences:  A006201 A006202 A006203 * A006205 A006206 A006207 KEYWORD nonn,nice,hard,more AUTHOR EXTENSIONS Additional comments and one more term from J. Hsiang, D. F. Hsu and Y. P. Shieh (arping(AT)turing.csie.ntu.edu.tw), Jun 03 2002 Corrected and extended by Roland Bacher, Dec 18 2007 Extended by Vesa Linja-aho (vesa.linja-aho(AT)tkk.fi), May 06 2009 STATUS approved

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