

A258377


O.g.f. satisfies A^2(z) = 1/(1  z)*( BINOMIAL(BINOMIAL(A(z))) ).


6



1, 3, 13, 79, 649, 6955, 93813, 1539991, 29884881, 669628819, 17005862301, 482399018527, 15108642099673, 517599894435643, 19247498583665029, 771922934908235751, 33206411983713679009, 1525025984109289947171, 74466779211331635306029, 3852255519421356879419631
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OFFSET

0,2


COMMENTS

For a fixed integer N, Hanna has considered the problem of finding an o.g.f. of the form A(z) = 1 + N*z + a(2)*z^2 + a(3)*z^3 + ..., with integer coefficients a(2), a(3), ... dependent on the parameter N, which is a solution to the functional equation A^(N+1) = ( BINOMIAL(A) )^N. Here BINOMIAL(F(z))= 1/(1  z)*F(z/(1  z)) denotes the binomial transform of the o.g.f. F(z).
The function A(z) is related to the triangle of ordered Stirling numbers A019538 via logarithmic differentiation. It can be shown that z*A'(z)/A(z) = Sum_{k >= 1} R(k,N)*z^k, where R(k,x) denotes the kth row polynomial of A019538; equivalently, A(z) = exp( Sum_{k >= 1} R(k,N)*z^k/k ).
Cases include A084784 (N = 1), A090352 (N = 2), A090355 (N = 3), A090357 (N = 4), A090362 (N = 5) and a signed version of A084785 (N = 2).
It turns out that the o.g.f. B(z) := A(z)^(1/N) also has integer coefficients. It satisfies the functional equation B^(N+1) = BINOMIAL(B^N). For cases see A084784 (N = 1), A090351 (N = 2), A090353 (N = 3), A090356 (N = 4), A090358 (N = 5) and A084784 (N = 2).
There are similar results to the above associated with triangle A145901, which can be viewed as a type B analog of A019538.
For a fixed integer N, consider the problem of finding an o.g.f. with integer coefficients (depending on the parameter N) of the form A(z) = 1 + N*z + a(2)*z^2 + a(3)*z^3 + ..., which is a solution to the functional equation A^(N+1)(z) = 1/(1  z) * ( BINOMIAL(BINOMIAL(A(z))) )^N; equivalently A^(N+1)(z) = 1/(1  z)* 1/(1  2*z)^N*A^N(z/(1  2*z)). This is the type B analog of Hanna's type A functional equation above.
It can be shown that z*A'(z)/A(z) = Sum_{k >= 1} P(k,N)*z^k, where P(k,x) denotes the kth row polynomial of A145901. However, unlike the type A situation, the type B function A(z)^(1/N) does not have all integer coefficients.
The present sequence is the case N = 1. For further examples of solutions to the type B functional equation see A258378 (N = 2), A258379 (N = 3), A258380 (N = 4) and A258381 (N = 5).
From Peter Bala, Dec 06 2017: (Start)
a(n) appears to be always odd. Calculation suggests that for k = 1,2,3,..., the sequence a(n) (mod 2^k) is purely periodic with period 2^(k1). For example, a(n) (mod 4) = (1,3,1,3,...) seems to be purely periodic with period 2 and a(n) (mod 8) = (1,3,5,7,1,3,5,7,...) seems to be purely periodic with period 4 (both checked up to n = 1000).
(End)


LINKS

Table of n, a(n) for n=0..19.
N. J. A. Sloane, Transforms.


FORMULA

a(0) = 1 and for n >= 1, a(n) = 1/n*Sum_{i = 0..n1} R(i+1,1)*a(n1i), where R(n,x) denotes the nth row polynomial of A145901.
O.g.f.: A(z) = 1 + 3*z + 13*z^2 + 79*z^3 + 649*z^4 + ... satisfies A^2(z) = 1/(1  z)*1/(1  2*z)*A(z/(1  2*z)).
O.g.f.: A(z) = exp( Sum_{k >= 1} R(k,1)*z^k/k ).
1 + z*A'(z)/A(z) = 1 + 3*z + 17*z^2 + 147*z^3 + 1697*z^4 + ... is the o.g.f. for A080253.


MAPLE

#A258377
with(combinat):
#recursively define row polynomials R(n, x) of A145901
R := proc (n, x) option remember; if n = 0 then 1 else 1 + x*add(binomial(n, i)*2^(ni)*R(i, x), i = 0..n1) end if; end proc:
#define a family of sequences depending on an integer parameter k
a := proc (n, k) option remember; if n = 0 then 1 else 1/n*add(R(i+1, k)*a(n1i, k), i = 0..n1) end if; end proc:
# display the case k = 1
seq(a(n, 1), n = 0..19);


MATHEMATICA

R[n_, x_] := R[n, x] = If[n==0, 1, 1+x*Sum[Binomial[n, i]*2^(ni)*R[i, x], {i, 0, n1}]];
a[n_, k_] := a[n, k] = If[n==0, 1, 1/n*Sum[R[i+1, k]*a[n1i, k], {i, 0, n1}]];
a[n_] := a[n, 1];
a /@ Range[0, 19] (* JeanFrançois Alcover, Oct 02 2019 *)


CROSSREFS

Cf. A019538, A080253, A084784, A084785, A090351, A090352, A090353, A090355, A090356, A090357, A090358, A090362, A145901, A258378 (N = 2), A258379 (N = 3), A258380 (N = 4), A258381 (N = 5).
Sequence in context: A010844 A090364 A112935 * A201795 A308521 A183278
Adjacent sequences: A258374 A258375 A258376 * A258378 A258379 A258380


KEYWORD

nonn,easy


AUTHOR

Peter Bala, May 28 2015


STATUS

approved



