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A189073
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Triangle read by rows: T(n,k) is the number of inversions in k-compositions of n for n >= 3, 2 <= k <= n-1.
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2
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1, 1, 3, 2, 6, 6, 2, 12, 18, 10, 3, 18, 42, 40, 15, 3, 27, 78, 110, 75, 21, 4, 36, 132, 240, 240, 126, 28, 4, 48, 204, 460, 600, 462, 196, 36, 5, 60, 300, 800, 1290, 1302, 812, 288, 45, 5, 75, 420, 1300, 2490, 3108, 2548, 1332, 405, 55, 6, 90, 570, 2000, 4440, 6594, 6692, 4608, 2070, 550, 66
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OFFSET
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3,3
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COMMENTS
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The Heibach et al. reference has a table for n <= 14.
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LINKS
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FORMULA
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EXAMPLE
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Triangle begins:
1;
1, 3;
2, 6, 6;
2, 12, 18, 10;
3, 18, 42, 40, 15;
3, 27, 78, 110, 75, 21;
4, 36, 132, 240, 240, 126, 28;
4, 48, 204, 460, 600, 462, 196, 36;
5, 60, 300, 800, 1290, 1302, 812, 288, 45;
5, 75, 420, 1300, 2490, 3108, 2548, 1332, 405, 55;
6, 90, 570, 2000, 4440, 6594, 6692, 4608, 2070, 550, 66;
...
T(5,3) = 6 because we have: 3+1+1, 1+3+1, 1+1+3, 2+2+1, 2+1+2, 1+2+2 having 2,1,0,2,1,0 inversions respectively. - Geoffrey Critzer, Mar 19 2014
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MAPLE
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T:= proc(n, k) option remember;
if k=2 then floor((n-1)/2)
elif k>=n then 0
else T(n-1, k) +k/(k-2) *T(n-1, k-1)
fi
end:
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MATHEMATICA
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T[n_, k_] := T[n, k] = Which[k == 2, Floor[(n-1)/2], k >= n, 0, True, T[n-1, k] + k/(k-2)*T[n-1, k-1]]; Table[Table[T[n, k], {k, 2, n-1}], {n, 3, 13}] // Flatten (* Jean-François Alcover, Jan 14 2014, after Alois P. Heinz *)
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CROSSREFS
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KEYWORD
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AUTHOR
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STATUS
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approved
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