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 A125625 Number of elements in the semigroup of type K_n. 0
 1, 2, 5, 18, 115, 1710, 83973, 22263378, 64146328635, 5387481983035854, 53332505278384935836485, 448356696524549059043145139274042, 52321110785739610206886887435107004491768788251, 4402157583106925818769478699470667613674438846830415891359277374958, 71336346872409035510345323533390649100576348044074421590685110464047512124710404684631077386973 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS The semigroup K_3 with 3 generators occurs in convexity theory; K_n is the generic semigroup with n generators. In the paper of Kudryavtseva and Mazorchuk it is shown that the n-th term of this sequence gives the number of words in the alphabet 1,2,...,n such that between any repetitions of any letter there must occur both a smaller and a bigger letter (in the natural order). For example, the word 2132 is allowed while 3213 is not. [V. Mazorchuk, Aug 24 2011] REFERENCES Alsaody, S. (2007). "Determining the Elements of a Semigroup." Uppsala, Sweden: Dept. Of Mathematics, Uppsala University, Report No. 2007:3. Kiselman, C. O. (2002). "A Semigroup of Operators in Convexity Theory." Trans. Am, Math. Soc., 354, No. 5, pp. 2035-2053. G. Kudryavtseva and V. Mazorchuk; On Kiselman's semigroup, Preprint Uppsala University 2005, published in: Yokohama Math. J. 55 (2009), no.1, 21-46. LINKS L. Forsberg, Effective representations of Hecke-Kiselman monoids of type A, arXiv preprint arXiv:1205.0676, 2012. - From N. J. A. Sloane, Oct 13 2012 M. Selin, Source code (C++) for algorithm. PROG (Magma) /* program for a(6) */ F := FreeMonoid(6); Q := quo< F | a^2 = a, b^2 = b, c^2 = c, d^2 = d, e^2 = e, f^2 = f, a*b*a = b*a*b = a*b, a*c*a = c*a*c = a*c, a*d*a = d*a*d = a*d, a*e*a = e*a*e = a*e, a*f*a = f*a*f = a*f, b*c*b = c*b*c = b*c, b*d*b = d*b*d = b*d, b*e*b = e*b*e = b*e, b*f*b = f*b*f = b*f, c*d*c = d*c*d = c*d, c*e*c = e*c*e = c*e, c*f*c = f*c*f = c*f, d*e*d = e*d*e = d*e, d*f*d = f*d*f = d*f, e*f*e = f*e*f = e*f >; M := RWSMonoid(Q); Order(M); /* Klaus Brockhaus, Mar 02 2007 */ CROSSREFS Sequence in context: A005805 A058338 A006896 * A281532 A097584 A197855 Adjacent sequences: A125622 A125623 A125624 * A125626 A125627 A125628 KEYWORD nonn AUTHOR Seidon Alsaody (Seidon.Alsaody.5527(AT)student.uu.se), Jan 27 2007 EXTENSIONS a(6) from Klaus Brockhaus, Mar 02 2007 More terms from M. Selin (mxrten(AT)kth.se), Jan 16 2008, Jan 25 2008 STATUS approved

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Last modified March 28 05:31 EDT 2023. Contains 361577 sequences. (Running on oeis4.)