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 A112091 Number of idempotent order-preserving partial transformations (of an n-element chain). 3
 1, 2, 6, 21, 76, 276, 1001, 3626, 13126, 47501, 171876, 621876, 2250001, 8140626, 29453126, 106562501, 385546876, 1394921876, 5046875001, 18259765626, 66064453126, 239023437501, 864794921876, 3128857421876, 11320312500001 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..1000 D. Callan, T. Mansour, Enumeration of small Wilf classes avoiding 1324 and two other 4-letter patterns, arXiv:1705.00933 [math.CO] (2017), Table 2 No 200 (offset 1 then). Laradji, A. and Umar, A. Combinatorial results for semigroups of order-preserving partial transformations, Journal of Algebra 278, (2004), 342-359. Laradji, A. and Umar, A. Combinatorial results for semigroups of order-decreasing partial transformations, J. Integer Seq. 7 (2004), 04.3.8 Index entries for linear recurrences with constant coefficients, signature (6,-10,5). FORMULA a(n)= ((sqrt(5))^(n-1))*(((sqrt(5)+1)/2)^n-((sqrt(5)-1)/2)^n)); a(n)=1+5*(a(n-1)-a(n-2)), a(0)=1, a(1)=2 G.f.: (2x-1)^2/((1-x)(1-5x+5x^2)). Convolution of A081567 with the sequence 1,-1,-1,-1 (-1 continued). [From R. J. Mathar, Sep 06 2008] a(n) = 1 +A030191(n-1). - R. J. Mathar, Jun 20 2011 a(0)=1, a(1)=2, a(2)=6, a(n)=6*a(n-1)-10*a(n-2)+5*a(n-3). Harvey P. Dale, Aug 20 2011 EXAMPLE a(2) = 6 because there are exactly 6 idempotent order-preserving partial transformations (on a 2-element chain), namely: the empty map, (1)->(1), (2)->(2), (1,2)->(1,1), (1,2)->(1,2), (1,2)->(2,2)- the mappings are coordinate-wise MATHEMATICA RecurrenceTable[{a[0]==1, a[1]==2, a[n]==1+5(a[n-1]-a[n-2])}, a[n], {n, 30}] (* or *) LinearRecurrence[{6, -10, 5}, {1, 2, 6}, 31] (* Harvey P. Dale, Aug 20 2011 *) PROG (MAGMA) [ n eq 1 select 1 else n eq 2 select 2 else n eq 3 select 6 else 6*Self(n-1)-10*Self(n-2)+ 5*Self(n-3): n in [1..30]]; // Vincenzo Librandi, Aug 21 2011 (PARI) Vec((2*x-1)^2/(1-x)/(1-5*x+5*x^2)+O(x^99)) \\ Charles R Greathouse IV, Aug 21 2011 CROSSREFS Sequence in context: A294819 A294820 A116782 * A108146 A116798 A279560 Adjacent sequences:  A112088 A112089 A112090 * A112092 A112093 A112094 KEYWORD nonn,easy AUTHOR Abdullahi Umar, Aug 25 2008 STATUS approved

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