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A003120
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Number of rooted trees with n nodes and omega-valency 1.
(Formerly M0836)
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6
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1, 1, 2, 3, 7, 13, 31, 66, 159, 365, 900, 2162, 5417, 13436, 34165, 86603, 223028, 574493, 1495524, 3900055, 10246172, 26982966, 71447432, 189664782, 505605729, 1351179886, 3623051567, 9737403960, 26243202664, 70878565004
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OFFSET
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1,3
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COMMENTS
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Draw the tree with the root at the bottom. The omega-valency of a leaf is 1; the omega-valency of any other vertex v is max(1,sum(omega-valence(s))-1) where the sum is over the vertices directly above v. Then the omega-valency of the tree itself is the omega-valency of the root. [F. Chapoton, Jul 25 2011; N. J. A. Sloane, Jul 27 2011]
Other names: Number of arborescences of type (n,1), or tapeworms.
Let phi_n denote the number of rooted trees on n nodes whose comparability graph is Hamiltonian. Then phi_1=1, phi_n = a(n-1) for n >= 2. [Arditti]
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REFERENCES
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N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
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LINKS
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F. Harary and R. W. Robinson, Tapeworms, Unpublished manuscript, circa 1973. (Annotated scanned copy)
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FORMULA
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The generating function is probably not rational. - F. Chapoton, Jul 26 2011
The g.f. -(z-1)*(3*z**2+z-1)/(-1+3*z+z**2-7*z**3+3*z**4) conjectured by Simon Plouffe in his 1992 dissertation is wrong (starting from index 11).
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EXAMPLE
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For n=4, the 3 rooted trees are
O O O
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MAPLE
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(Maple program from N. J. A. Sloane, Jul 27 2011, based on Eq. (2) of the Arditti paper. This proceeds in very small steps because I was trying to isolate the error in that formula. The error turns out to be in the display following (2): this is not phi(x). Otherwise Eq. (2) is correct.)
S:=x*y + x^2*y + 2*x^3*y + x^4*(3*y+y^2) + x^5*(7*y+y^2+y^3);
M:=30;
for n from 6 to M do
t5:=series(series(S, y, n), x, n+1);
t6:=add( subs(x=x^k, subs(y=y^k, t5))/k, k=1..n+1);
t7:=series(series(t6, y, n), x, n+1);
t8:=(x/y)*(exp(t7)-1);
t9:=series(series(t8, y, n), x, n+1);
xf1:=subs(y=0, series(t5/y, y, n));
t10:=series(series(xf1, y, n), x, n+1);
t11:=series(series(t9-x*t10, y, n), x, n+1);
t12:=series(series(t11+x*y*t10+x*y, y, n), x, n+1);
t13:=coeff(t12, x, n);
S:=S+x^n*t13;
od:
xf1:=subs(y=0, series(S/y, y, M+1));
series(%, x, M+1);
seriestolist(%);
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PROG
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(Sage)
a = polygen(QQ, 'a')
an = FractionField(a.parent())
ri = PowerSeriesRing(an, 'x')
x = ri.gen()
t = ri.zero().O(1)
v = ri.zero().O(1)
for l in range(n):
truc = ri.zero()
for k in range(1, l + 1):
truc += ri([u(a=a**k) for u in t(x**k).truncate(l+1)]) / k
t = a*x+x*v+x*(t-v)/a-x/a*(t+1)+x*(exp(truc))/a
v = a*ri([u(a=0) for u in t/a])
return (v / a).coefficients()
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CROSSREFS
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KEYWORD
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nonn,nice,easy
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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