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A010094
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Triangle of Euler-Bernoulli or Entringer numbers.
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9
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1, 1, 1, 2, 2, 1, 5, 5, 4, 2, 16, 16, 14, 10, 5, 61, 61, 56, 46, 32, 16, 272, 272, 256, 224, 178, 122, 61, 1385, 1385, 1324, 1202, 1024, 800, 544, 272, 7936, 7936, 7664, 7120, 6320, 5296, 4094, 2770, 1385, 50521, 50521, 49136, 46366, 42272, 36976, 30656, 23536, 15872, 7936, 353792
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OFFSET
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1,4
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COMMENTS
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T(n, k) is the number of up-down permutations of n starting with k where 1 <= k <= n. - Michael Somos, Jan 20 2020
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REFERENCES
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R. C. Entringer, A combinatorial interpretation of the Euler and Bernoulli numbers, Nieuw Archief voor Wiskunde, 14 (1966), 241-246.
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LINKS
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J. Millar, N. J. A. Sloane and N. E. Young, A new operation on sequences: the Boustrophedon transform, J. Combin. Theory, 17A (1996) 44-54 (Abstract, pdf, ps).
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FORMULA
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T(1, 1) = 1; T(n, n) = 0 if n > 1; T(n, k) = T(n, k+1) + T(n-1, n-k) if 1 <= k < n. - Michael Somos, Jan 20 2020
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EXAMPLE
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Triangle begins:
1;
1, 1;
2, 2, 1;
5, 5, 4, 2;
16, 16, 14, 10, 5;
61, 61, 56, 46, 32, 16;
272, 272, 256, 224, 178, 122, 61;
1385, 1385, 1324, 1202, 1024, 800, 544, 272;
7936, 7936, 7664, 7120, 6320, 5296, 4094, 2770, 1385;
... (End)
Up-down permutations for n = 4 are k = 1: 1324, 1423; k = 2: 2314, 2413; k = 3: 3411; k = 4: none. - Michael Somos, Jan 20 2020
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MAPLE
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b:= proc(u, o) option remember; `if`(u+o=0, 1,
add(b(o-1+j, u-j), j=1..u))
end:
T:= (n, k)-> b(n-k+1, k-1):
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MATHEMATICA
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e[0, 0] = 1; e[_, 0] = 0; e[n_, k_] := e[n, k] = e[n, k-1] + e[n-1, n-k]; Join[{1}, Table[e[n, k], {n, 0, 11}, {k, n, 1, -1}] // Flatten] (* Jean-François Alcover, Aug 13 2013 *)
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PROG
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(PARI) {T(n, k) = if( n < 1 || k >= n, k == 1 && n == 1, T(n, k+1) + T(n-1, n-k))}; /* Michael Somos, Jan 20 2020 */
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CROSSREFS
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KEYWORD
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AUTHOR
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EXTENSIONS
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More terms from Will Root (crosswind(AT)bright.net), Oct 08 2001
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STATUS
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approved
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