A325206 - OEIS

A325206

a(n) is the number of labeled rooted trees on a set of size n where each node has at most 7 neighbors that are further away from the root than the node itself.

0, 1, 2, 9, 64, 625, 7776, 117649, 2097152, 43046712, 999999180, 25937373990, 743005653984, 23297946618804, 793707788417544, 29192570114517810, 1152902963147295360, 48660197610533102880, 2185856466420637543104, 104124189019562479248624, 5242691958381764070687360

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OFFSET

0,3

COMMENTS

A preimage constraint on a function is a set of nonnegative integers such that the size of the inverse image of any element is one of the values in that set. View a labeled rooted tree as an endofunction on the set {1,2,...,n} by sending every non-root node to its neighbor that is closer to the root and sending the root to itself. Thus, a(n) is the number of endofunctions on a set of size n with exactly one cyclic point and such that each preimage has at most 7 entries.

LINKS

Table of n, a(n) for n=0..20.

B. Otto, Coalescence under Preimage Constraints, arXiv:1903.00542 [math.CO], 2019, Corollaries 5.3 and 7.8.

FORMULA

a(n) = (n-1)! * [x^(n-1)] e_7(x)^n, where e_k(x) is the truncated exponential 1 + x + x^2/2! + ... + x^k/k!. The link above yields explicit constants c_k, r_k so that the columns are asymptotically c_7 * n^(-3/2) * r_7^-n.

PROG

(Python)

# print first num_entries entries in the sequence

import math, sympy; x=sympy.symbols('x')

k=7; num_entries = 64

P=range(k+1); eP=sum([x**d/math.factorial(d) for d in P]); r = [0, 1]; curr_pow = eP

for term in range(1, num_entries-1):

curr_pow=(curr_pow*eP).expand()

r.append(curr_pow.coeff(x**term)*math.factorial(term))

print(r)

CROSSREFS

Column k=7 of A325201; see that entry for sequences related to other preimage constraints constructions.