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A324806 a(n) is the number of endofunctions on a set of size n with preimage constraint {0, 1, 2, 3, 4, 5, 6}.

%I A324806 #18 Oct 03 2024 14:07:02
%S A324806 1,1,4,27,256,3125,46656,823536,16776760,387399096,9999087840,
%T A324806 285273493890,8914479240744,302804132739108,11108778138153696,
%U A324806 437740674563572380,18439146872674688160,826846479804875930400,39325078741869629444736,1977213223548343109320992
%N A324806 a(n) is the number of endofunctions on a set of size n with preimage constraint {0, 1, 2, 3, 4, 5, 6}.
%C A324806 A preimage constraint 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.
%C A324806 Thus, the n-th term of the sequence is the number of endofunctions on a set of size n such that each preimage has at most 6 elements. Equivalently, it is the number of n-letter words from an n-letter alphabet such that no letter appears more than 6 times.
%H A324806 Benjamin Otto, <a href="https://arxiv.org/abs/1903.00542">Coalescence under Preimage Constraints</a>, arXiv:1903.00542 [math.CO], 2019, Corollaries 5.6 and 7.8.
%F A324806 a(n) = n! * [x^n] e_6(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_6 * n^(-1/2) * r_6^-n.
%p A324806 b:= proc(n, i) option remember; `if`(n=0 and i=0, 1, `if`(i<1, 0,
%p A324806       add(b(n-j, i-1)*binomial(n, j), j=0..min(6, n))))
%p A324806     end:
%p A324806 a:= n-> b(n$2):
%p A324806 seq(a(n), n=0..20);  # _Alois P. Heinz_, Apr 01 2019
%t A324806 b[n_, i_] := b[n, i] = If[n == 0 && i == 0, 1, If[i<1, 0, Sum[b[n-j, i-1]* Binomial[n, j], {j, 0, Min[6, n]}]]];
%t A324806 a[n_] := b[n, n];
%t A324806 Table[a[n], {n, 0, 20}] (* _Jean-François Alcover_, May 31 2019, after _Alois P. Heinz_ *)
%o A324806 (Python)
%o A324806 # print first num_entries entries in the sequence
%o A324806 import math, sympy; x=sympy.symbols('x')
%o A324806 k=6; num_entries = 64
%o A324806 P=range(k+1); eP=sum([x**d/math.factorial(d) for d in P]); r = [1]; curr_pow = 1
%o A324806 for term in range(1,num_entries):
%o A324806     curr_pow=(curr_pow*eP).expand()
%o A324806     r.append(curr_pow.coeff(x**term)*math.factorial(term))
%o A324806 print(r)
%Y A324806 Column k=6 of A306800; see that entry for sequences related to other preimage constraints constructions.
%K A324806 easy,nonn
%O A324806 0,3
%A A324806 _Benjamin Otto_, Mar 25 2019