OFFSET
0,2
COMMENTS
Binomial transform of A000225 (if this starts 1,1,3,7....).
Let P(A) be the power set of an n-element set A. Then a(n) = the number of pairs of elements {x,y} of P(A) for which either 0) x and y are intersecting and for which either x is a proper subset of y or y is a proper subset of x, or 1) x = y. - Ross La Haye, Jan 10 2008
Let P(A) be the power set of an n-element set A and R be a relation on P(A) such that for all x, y of P(A), xRy if either 0) x is not a subset of y and y is not a subset of x and x and y are disjoint, or 1) x equals y. Then a(n) = |R|. - Ross La Haye, Mar 19 2009
LINKS
M. H. Albert, M. D. Atkinson, and V. Vatter, Inflations of geometric grid classes: three case studies, arXiv:1209.0425 [math.CO], 2012.
Ross La Haye, Binary Relations on the Power Set of an n-Element Set, Journal of Integer Sequences, Vol. 12 (2009), Article 09.2.6.
Jay Pantone, The Enumeration of Permutations Avoiding 3124 and 4312, arXiv:1309.0832 [math.CO], 2013.
Index entries for linear recurrences with constant coefficients, signature (6,-11,6).
FORMULA
G.f.: (1-4*x+5*x^2)/((1-x)*(1-2*x)*(1-3*x)).
E.g.f.: exp(3*x) - exp(2*x) + exp(x).
Row sums of triangle A134319. - Gary W. Adamson, Oct 19 2007
a(n) = 2*StirlingS2(n+1,3) + StirlingS2(n+1,2) + 1. - Ross La Haye, Jan 10 2008
EXAMPLE
From Gus Wiseman, Dec 10 2019: (Start)
Also the number of achiral set-systems on n vertices, where a set-system is achiral if it is not changed by any permutation of the covered vertices. For example, the a(0) = 1 through a(3) = 20 achiral set-systems are:
0 0 0 0
{1} {1} {1}
{2} {2}
{12} {3}
{1}{2} {12}
{1}{2}{12} {13}
{23}
{123}
{1}{2}
{1}{3}
{2}{3}
{1}{2}{3}
{1}{2}{12}
{1}{3}{13}
{2}{3}{23}
{12}{13}{23}
{1}{2}{3}{123}
{12}{13}{23}{123}
{1}{2}{3}{12}{13}{23}
{1}{2}{3}{12}{13}{23}{123}
BII-numbers of these set-systems are A330217. Fully chiral set-systems are A330282, with covering case A330229.
(End)
MATHEMATICA
LinearRecurrence[{6, -11, 6}, {1, 2, 6}, 30] (* G. C. Greubel, Feb 13 2019 *)
PROG
(PARI) a(n)=3^n-2^n+1 \\ Charles R Greathouse IV, Oct 07 2015
(Magma) [3^n-2^n+1: n in [0..30]]; // G. C. Greubel, Feb 13 2019
(Sage) [3^n-2^n+1 for n in range(30)] # G. C. Greubel, Feb 13 2019
(GAP) List([0..30], n -> 3^n-2^n+1); # G. C. Greubel, Feb 13 2019
KEYWORD
nonn,easy
AUTHOR
Paul Barry, Apr 27 2003
STATUS
approved