OFFSET
1,3
COMMENTS
A weak inversion is a pair of cells (i,j) with i<j where j appears weakly below and weakly left of i. [Joerg Arndt, Feb 18 2014]
LINKS
Alois P. Heinz, Table of n, a(n) for n = 1..50
M. Shynar, On Inversions in Standard Young Tableaux
Wikipedia, Young tableau
MAPLE
b:= proc(l) option remember; `if`({l[]}={0}, [1, 0],
add(`if`(l[j]>`if`(j=1, 0, l[j-1]), (f->f+[0, f[1]*
add(l[h]-l[j]+1, h=j+1..nops(l))])
(b(subsop(j=l[j]-1, l))), 0), j=1..nops(l)))
end:
g:= proc(n, i, l) `if`(n=0 or i=1, b([1$n, l[]]),
`if`(i<1, 0, g(n, i-1, l)+
`if`(i>n, 0, g(n-i, i, [i, l[]]))))
end:
a:= n-> g(n$2, [])[2]:
seq(a(n), n=1..23); # Alois P. Heinz, Aug 09 2013
MATHEMATICA
inversions[t_?TableauQ]:=Block[{t0}, t0=(First[Position[t, #1]]&) /@ Range[Max[t]]; Cases[Table[{i, j}, {j, 2, Max[t]}, {i, j-1}], {i_, j_}/; MatchQ[t0[[i]]-t0[[j]], {_?Negative, _?Positive}]->{i, j}, {2}]];
Table[Tr[Length[weakinversions[#]]& /@ Tableaux[n]], {n, 12}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Wouter Meeussen, Aug 04 2013
EXTENSIONS
Terms verified and more terms added, Joerg Arndt, Aug 07 2013
a(19)-a(25) from Alois P. Heinz, Aug 08 2013
STATUS
approved