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 A178519 Triangle read by rows: T(n,k) is the number of ordered trees with n edges and having k vertices of outdegree 2 that have (two) leaves as their (two) children. 1
 1, 1, 1, 1, 4, 1, 11, 3, 32, 10, 98, 33, 1, 309, 114, 6, 998, 402, 30, 3285, 1439, 137, 1, 10981, 5205, 600, 10, 37178, 18976, 2562, 70, 127227, 69610, 10758, 416, 1, 439369, 256626, 44640, 2250, 15, 1529280, 949974, 183594, 11452, 140, 5359314, 3528725 (list; graph; refs; listen; history; text; internal format)
 OFFSET 0,5 COMMENTS Row n has 1+floor(n/3) entries (n>=3). Sum of entries in row n is A000108(n) (Catalan numbers). T(n,0)=A178520(n). Sum(k*T(n,k), k>=0)=binom(2n-5,n-2)=A001700(n-3). Statistic suggested by Lou Shapiro. LINKS FORMULA G.f. G=G(t,z) satisfies zG^2-(1-z^3+tz^3)G+1-z^2+tz^2=0. EXAMPLE T(2,1)=1 because we have \/ . Triangle starts: 1; 1; 1,1; 4,1; 11,3; 32,10; 98,33,1; MAPLE eq := z*G^2-(1-z^3+t*z^3)*G+1-z^2+t*z^2: G := RootOf(eq, G): Gser := simplify(series(G, z = 0, 18)): for n from 0 to 15 do P[n] := sort(coeff(Gser, z, n)) end do: 1; 1; 1, 1; for n from 3 to 15 do seq(coeff(P[n], t, j), j = 0 .. floor((1/3)*n)) end do; # yields sequence in triangular form CROSSREFS Cf. A000108, A178520, A001700 Sequence in context: A111964 A242351 A124324 * A094503 A113897 A158753 Adjacent sequences:  A178516 A178517 A178518 * A178520 A178521 A178522 KEYWORD nonn,tabf AUTHOR Emeric Deutsch, May 31 2010 STATUS approved

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Last modified July 17 09:15 EDT 2019. Contains 325099 sequences. (Running on oeis4.)