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 A034928 Triangle of ballot numbers. 2
 1, 1, 1, 1, 1, 1, 1, 2, 2, 1, 1, 3, 4, 4, 1, 1, 4, 6, 9, 9, 1, 1, 5, 8, 15, 21, 21, 1, 1, 6, 10, 22, 36, 51, 51, 1, 1, 7, 12, 30, 54, 91, 127, 127, 1, 1, 8, 14, 39, 75, 142, 232, 323, 323, 1, 1, 9, 16, 49, 99, 205, 370, 603, 835, 835, 1, 1, 10, 18, 60, 126, 281, 545 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,8 LINKS Reinhard Zumkeller, Rows n = 0..125 of triangle, flattened M. Aigner, Motzkin Numbers, Europ. J. Comb. 19 (1998), 663-675. FORMULA a(0, 0)=a(0, 1)=1, a(n, n+1)=a(n, n), a(n, k)=a(n-1, 0)+...+a(n-1, k-2)+a(n-1, k) (n >= 1, 0<=k<=n). Or, from David W. Wilson: a(n, 0) = 1; a(n, 1) = 1; a(n, 2) = n; a(n, k) = 0 if k > n+1; a(n, k) = a(n-1, k) + a(n, k-1) + a(n-1, k-2) - a(n-1, k-1) otherwise. EXAMPLE Triangle begins 1, 1, 1, 1, 1, 1, 1, 2, 2, 1, 1, 3, 4, 4, 1, 1, 4, 6, 9, 9, 1, 1, 5, 8, 15, 21, 21, 1, 1, 6, 10, 22, 36, 51, 51, 1, 1, 7, 12, 30, 54, 91, 127, 127, 1, 1, 8, 14, 39, 75, 142, 232, 323, 323, 1, 1, 9, 16, 49, 99, 205, 370, 603, 835, 835, ... MATHEMATICA a[n_, 0] := 1; a[n_, 1] := 1; a[n_, 2] := n; a[n_, k_] := If [k > n + 1, 0, a[n - 1, k] + a[n, k - 1] + a[n - 1, k - 2] - a[n - 1, k - 1]]; Grid[Table[a[n, k], {n, 0, 10}, {k, 0, n + 1}]] (* Replace Grid with Flatten to get the sequence. *) (* L. Edson Jeffery, Aug 02 2014 (after David W. Wilson) *) PROG (Haskell) a034928 n k = a034928_tabf !! n !! k a034928_row n = a034928_tabf !! n a034928_tabf = iterate f [1, 1] where    f us = vs ++ [last vs] where           vs = zipWith (+) us (0 : scanl (+) 0 us) -- Reinhard Zumkeller, Sep 20 2014 CROSSREFS Right-hand edge is A001006. Cf. A247364 (mirrored). Sequence in context: A248736 A292508 A237597 * A280267 A161671 A144444 Adjacent sequences:  A034925 A034926 A034927 * A034929 A034930 A034931 KEYWORD nonn,tabf,easy AUTHOR EXTENSIONS More terms from David W. Wilson. STATUS approved

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Last modified June 19 23:01 EDT 2019. Contains 324222 sequences. (Running on oeis4.)