The OEIS Foundation is supported by donations from users of the OEIS and by a grant from the Simons Foundation.

 Hints (Greetings from The On-Line Encyclopedia of Integer Sequences!)
 A055580 Björner-Welker sequence: 2^n*(n^2 + n + 2) - 1. 13
 1, 7, 31, 111, 351, 1023, 2815, 7423, 18943, 47103, 114687, 274431, 647167, 1507327, 3473407, 7929855, 17956863, 40370175, 90177535, 200278015, 442499071, 973078527, 2130706431, 4647288831, 10099884031, 21877489663 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,2 COMMENTS a(n) is the d=1 Betti number of the complement of '3-equal' arrangements in n dimensional real space, see Björner-Welker reference, Table I, pp. 308-309, column '1' with k=3 and Th. 5.2, pp. 297-298. Binomial transform of [1/2, 2/3, 3/4, 4/5, ...] = 1/2, 7/6, 31/12, 111/20, 351/30, 1023/42, ..., where 2, 6, 12, 20, ... = A002378 (deleting the zero). - Gary W. Adamson, Apr 28 2005 Number of three-dimensional block structures associated with n joint systems in the construction of stable underground structures. - Richard M. Green, Jul 26 2011 Number of monotone mappings from the chain with three points to the complete binary tree of height n (n+1 levels). For example, the seven monotone mappings from the chain with three points (denoted 1,2,3, in order) to the complete binary tree with two levels (with a the root of the tree, and b, c the atoms) are: f(1)=f(2)=f(3)=a; f(1)=f(2)=a, f(3)=b; f(1)=f(2)=a, f(3)=c; f(1)=a, f(2)=f(3)=b; f(1)=a, f(2)=f(3)=c; f(1)=f(2)=f(3)=b; f(1)=f(2)=f(3)=c. - Pietro Codara, Mar 26 2015 REFERENCES H. Barcelo and S. Smith, The discrete fundamental group of the order complex of B_n, Abstract 1020-05-141, 1020th Meeting Amer. Math. Soc., Cincinatti, Ohio, Oct 21-22, 2006. LINKS Vincenzo Librandi, Table of n, a(n) for n = 0..1000 H. Barcelo and R. Laubenbacher, Perspectives on A-homotopy theory and its applications, Discr. Math., 298 (2005), 39-61. H. Barcelo and S. Smith, The discrete fundamental group of the order complex of B_n, arXiv:0711.0915 [math.CO], 2007. A. Björner and V. Welker, The homology of "k-equal" manifolds and related partition lattices, Adv. Math., 110 (1995), 277-313. Harry Crane, Left-right arrangements, set partitions, and pattern avoidance, Australasian Journal of Combinatorics, 61(1) (2015), 57-72. Robert Davis, Greg Simay, Further Combinatorics and Applications of Two-Toned Tilings, arXiv:2001.11089 [math.CO], 2020. G.G. Kocharyan and A.M. Kulyukin, Construction of a three-dimensional block structure on the basis of jointed rock parameters estimating the stability of underground workings, Soil Mech. Found. Eng., 31 (1994), 62-66. A. F. Y. Zhao, Pattern Popularity in Multiply Restricted Permutations, Journal of Integer Sequences, 17 (2014), #14.10.3. Index entries for linear recurrences with constant coefficients, signature (7,-18,20,-8). FORMULA a(n) = A055252(n+3, 3). a(n) = Sum_{j=0..n-1} a(j) + A045618(n), n >= 1. G.f.: 1/((1-2*x)^3*(1-x)). Partial sums of A001788 (without leading zero). - Paul Barry, Jun 26 2003 a(n) = A001788(n) - A000337(n). - Jon Perry, Dec 12 2003 a(n) = A119258(n+4,n). - Reinhard Zumkeller, May 11 2006 E.g.f.: 2*(1 + 2*x + 2*x^2)*exp(2*x) - exp(x). - G. C. Greubel, Oct 28 2016 a(n) = Sum_{k=0..n+1} Sum_{i=0..n+1} i^2 * C(k,i). - Wesley Ivan Hurt, Sep 21 2017 MATHEMATICA Table[ n*(n+1)*2^(n-2), {n, 0, 26}] // Accumulate // Rest (* Jean-François Alcover, Jul 09 2013, after Paul Barry *) LinearRecurrence[{7, -18, 20, -8}, {1, 7, 31, 111}, 30] (* Harvey P. Dale, Nov 27 2014 *) PROG (MAGMA) [2^n*(n^2+n+2)-1: n in [0..35]]; // Vincenzo Librandi, Jul 28 2011 (PARI) a(n)=(n^2+n+2)<

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent
The OEIS Community | Maintained by The OEIS Foundation Inc.

Last modified October 27 17:25 EDT 2020. Contains 338035 sequences. (Running on oeis4.)