OFFSET
1,2
COMMENTS
a(n) is the number of corners in the set of tree-like tableaux of size n (see Gao et al. link). - Michel Marcus, Nov 18 2015
LINKS
G. C. Greubel, Table of n, a(n) for n = 1..445
Alice L.L. Gao, Emily X.L. Gao, Patxi Laborde-Zubieta, Brian Y. Sun, Enumeration of Corners in Tree-like Tableaux, arXiv:1511.05456 [math.CO], 2016.
Ira M. Gessel, Generating functions and enumeration of sequences, Ph. D. Thesis, MIT, 1977.
FORMULA
a(n) = n!*(n+4)/6 for n >= 2.
E.g.f.: x*(6-6*x+x^2)/(6*(1-x)^2).
EXAMPLE
a(3) = 7 because there are 7 runs of length 1 in the permutations 123, 13(2), (2)13, 23(1), (3)12, (3)(2)(1) (shown between parentheses).
MAPLE
seq(ceil(n!*(n+4)/6), n=1..23);
MATHEMATICA
Join[{1}, Table[n! (n + 4)/6, {n, 2, 20}]] (* Vincenzo Librandi, Nov 18 2015 *)
Rest[With[{nmax = 50}, CoefficientList[Series[x*(6 - 6*x + x^2)/(6*(1 - x)^2), {x, 0, nmax}], x]*Range[0, nmax]!]] (* G. C. Greubel, Dec 20 2017 *)
PROG
(Magma) [1] cat [Factorial(n)*(n+4)/6: n in [2..25]]; // Vincenzo Librandi, Nov 18 2015
(PARI) x='x+O('x^30); Vec(serlaplace(x*(6-6*x+x^2)/(6*(1-x)^2))) \\ G. C. Greubel, Dec 20 2017
(PARI) a(n) = if(n==1, 1, n!*(n+4)/6); \\ Altug Alkan, Dec 21 2017
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Emeric Deutsch and Ira M. Gessel, Sep 03 2004
STATUS
approved