login
This site is supported by donations to The OEIS Foundation.

 

Logo


Hints
(Greetings from The On-Line Encyclopedia of Integer Sequences!)
A098777 Pseudo-factorials: a(0)=1, a(n+1) = (-1)^(n+1) * Sum_{k=0..n} binomial(n,k) * a(k)*a(n-k), n>=0. 4
1, -1, -2, 2, 16, -40, -320, 1040, 12160, -52480, -742400, 3872000, 66457600, -411136000, -8202444800, 58479872000, 1335009280000, -10791497728000, -277035646976000, 2502527565824000, 71391934873600000, -712816377856000000, -22367684235100160000, 244597236078018560000 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,3

COMMENTS

A variation on the usual factorials (which satisfy the recursion (n+1)!=sum('binomial(n,k)*k!*(n-k)!','k'=0..n) for n>=0).

This sequence seems to satisfy an analog of Wilson's Theorem (which states that (p-1)! equals -1 modulo p for p a prime): For p<10000 a prime congruent to 2 modulo 3, we have a(p-1) congruent to 1 mod p and a(n) congruent to 0 mod p for n>p. For p<10000 a prime congruent to 1 mod 3 we have a(p-1)+a(p) congruent to -1 modulo p.

On the analytic side, the sequence is closely related (via its exponential generating series) to the elliptic curve of j-invariant O (corresponding to the regular hexagonal lattice).

This sequence has a generating function expressible in terms of the Dixon elliptic function sm(x,0) whose coefficients are A104133. The ordinary generating function has a continued fraction expansion of Jacobi type: the numerators are -j^2*(2-(-1)^j) and the denominators are (-1)^(j-1)(j+1/2+(-1)^j/2). - Philippe Flajolet and Roland Bacher, Jan 18 2009

LINKS

T. D. Noe, Table of n, a(n) for n = 0..100

R. Bacher and P. Flajolet, Pseudo-factorials, Elliptic Functions and Continued Fractions, arXiv:0901.1379 [math.CA], 2009.

P. Flajolet, Research Papers

FORMULA

The exponential generating function f(z)=sum('a(n)*z^n/n!', 'n'=0..infinity) satisfies f'(z)=-f(-z)^2 and is an elliptic function with respect to a regular hexagonal lattice (moreover, -f(z)f(-z) is (up to translation) a Weierstrass function.

a(n) = -n!/R^(n+1)*sum(b^(8*p+4*q)/((p-1/2)*b+(q-1/2)/b)^(n+1), p = -infinity..infinity, q = -infinity..infinity), where b = exp(I*Pi/6) and R = 2^(-4/3)/Pi*GAMMA(1/3)^3. - Philippe Flajolet and Roland Bacher, Jan 18 2009

G.f.: 1/Q(0), where Q(k) = 1 + (2*k+1)*x  +  3*x^2*(2*k+1)^2/(1 - (2*k+1)*x  +  x^2*(2*k+2)^2/Q(k+1) ); (continued fraction after P. Flajolet). - Sergei N. Gladkovskii, Dec 05 2013

E.g.f.: (1 - 3*Series_Reversion( Integral 1/(1 - 9*x^2)^(2/3) dx ))^(1/3). - Paul D. Hanna, Apr 10 2014

EXAMPLE

G.f. = 1 - x - 2*x^2 + 2*x^3 + 16*x^4 - 40*x^5 - 320*x^6 + 1040*x^7 + ...

MATHEMATICA

max = 23; f[z_] = Sum[a[n]*(z^n/n!), {n, 0, max}]; a[0] = 1; a[1] = -1; eq = Rest[ Thread[ CoefficientList[f'[z] + f[-z]^2, z] == 0]]; sol = Solve[ Drop[eq, -max-1]][[1]]; Table[a[n], {n, 0, max}] /. sol (* Jean-François Alcover, Oct 05 2011 *)

a[0] = 1; a[n_] := a[n] = (-1)^n*Sum[Binomial[n-1, k]*a[k]*a[n-k-1], {k, 0, n-1}]; Table[a[n], {n, 0, 23}] (* Jean-François Alcover, Apr 15 2015 *)

PROG

(PARI) a(n)=local(A=1); A=(1-3*serreverse(intformal(1/(1-9*x^2 +x*O(x^n))^(2/3))))^(1/3); n!*polcoeff(A, n)

for(n=0, 20, print1(a(n), ", ")) \\ Paul D. Hanna, Apr 10 2014

CROSSREFS

Cf. A144689, A144750, A235166.

Sequence in context: A177832 A230800 A076615 * A127226 A001119 A216387

Adjacent sequences:  A098774 A098775 A098776 * A098778 A098779 A098780

KEYWORD

sign

AUTHOR

Roland Bacher, Oct 04 2004

STATUS

approved

Lookup | Welcome | Wiki | Register | Music | Plot 2 | Demos | Index | Browse | More | WebCam
Contribute new seq. or comment | Format | Style Sheet | Transforms | Superseeker | Recent | More pages
The OEIS Community | Maintained by The OEIS Foundation Inc.

License Agreements, Terms of Use, Privacy Policy .

Last modified September 26 06:24 EDT 2017. Contains 292502 sequences.