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A214298
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Number of n-th order connected Feynman diagrams.
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4
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1, 4, 80, 3552, 271104, 31342080, 5087692800, 1102119137280, 307098445086720, 107024623079915520, 45623072462929920000, 23361142846626437529600, 14152553458020435728793600, 10014464602019135560561459200, 8186110540477899728656269312000
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OFFSET
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0,2
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LINKS
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FORMULA
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MAPLE
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b:= proc(x, y, t) option remember; `if`(y>x or y<0, 0,
`if`(x=0, 1, b(x-1, y-1, false)*`if`(t, (x+y)/y, 1) +
b(x-1, y+1, true) ))
end:
a:= n-> n!*2^n*b(2*n, 0, false):
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MATHEMATICA
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b[x_, y_, t_] := b[x, y, t] = If[y>x || y<0, 0, If[x == 0, 1, b[x-1, y-1, False] * If[t, (x+y)/y, 1] + b[x-1, y+1, True]]]; a[n_] := n!*2^n*b[2*n, 0, False]; Table[a[n], {n, 0, 15}] (* Jean-François Alcover, Jun 22 2015, after Alois P. Heinz *)
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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