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A320222 Number of unlabeled rooted trees with n nodes in which the non-leaf branches directly under any given node are all equal. 12
1, 1, 2, 4, 9, 18, 39, 78, 161, 324, 658, 1316, 2657, 5314, 10668, 21347, 42777, 85554, 171290, 342580, 685498, 1371037, 2742733, 5485466, 10972351, 21944711, 43892080, 87784323, 175574004, 351148008, 702307038, 1404614076, 2809249582, 5618499824, 11237042426 (list; graph; refs; listen; history; text; internal format)
OFFSET

1,3

COMMENTS

This is a weaker condition than achirality (cf. A003238).

LINKS

Andrew Howroyd, Table of n, a(n) for n = 1..500

FORMULA

a(n) = 1 + Sum_{k = 2...n-1} floor((n-1)/k) * a(k).

a(n) ~ c * 2^n, where c = 0.3270422384018894564479397100499014525700668391191792769625407295138546463... - Vaclav Kotesovec, Sep 07 2019

EXAMPLE

The a(1) = 1 through a(6) = 18 rooted trees:

  o  (o)  (oo)   (ooo)    (oooo)     (ooooo)

          ((o))  ((oo))   ((ooo))    ((oooo))

                 (o(o))   (o(oo))    (o(ooo))

                 (((o)))  (oo(o))    (oo(oo))

                          (((oo)))   (ooo(o))

                          ((o)(o))   (((ooo)))

                          ((o(o)))   ((o(oo)))

                          (o((o)))   ((oo(o)))

                          ((((o))))  (o((oo)))

                                     (o(o)(o))

                                     (o(o(o)))

                                     (oo((o)))

                                     ((((oo))))

                                     (((o)(o)))

                                     (((o(o))))

                                     ((o((o))))

                                     (o(((o))))

                                     (((((o)))))

MATHEMATICA

saue[n_]:=Sum[If[SameQ@@DeleteCases[ptn, 1], If[DeleteCases[ptn, 1]=={}, 1, saue[DeleteCases[ptn, 1][[1]]]], 0], {ptn, IntegerPartitions[n-1]}];

Table[saue[n], {n, 15}]

PROG

(PARI) seq(n)={my(v=vector(n)); for(n=1, n, v[n] = 1 + sum(k=2, n-1, (n-1)\k*v[k])); v} \\ Andrew Howroyd, Oct 26 2018

CROSSREFS

Cf. A002541, A003238, A010766, A126656, A014668, A167865, A214577, A316782, A317099, A317100, A317712, A320230.

Sequence in context: A327739 A018000 A329668 * A036610 A219755 A289846

Adjacent sequences:  A320219 A320220 A320221 * A320223 A320224 A320225

KEYWORD

nonn

AUTHOR

Gus Wiseman, Oct 07 2018

STATUS

approved

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Last modified March 30 15:42 EDT 2020. Contains 333127 sequences. (Running on oeis4.)