

A122848


Exponential Riordan array (1,x(1+x/2)).


9



1, 0, 1, 0, 1, 1, 0, 0, 3, 1, 0, 0, 3, 6, 1, 0, 0, 0, 15, 10, 1, 0, 0, 0, 15, 45, 15, 1, 0, 0, 0, 0, 105, 105, 21, 1, 0, 0, 0, 0, 105, 420, 210, 28, 1, 0, 0, 0, 0, 0, 945, 1260, 378, 36, 1, 0, 0, 0, 0, 0, 945, 4725, 3150, 630, 45, 1, 0, 0, 0, 0, 0, 0, 10395, 17325, 6930, 990, 55, 1, 0, 0
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OFFSET

0,9


COMMENTS

Entries are Bessel polynomial coefficients. Row sums are A000085. Diagonal sums are A122849. Inverse is A122850. Product of A007318 and A122848 gives A100862.
T(n,k) is the number of self inverse permutations of {1,2,...,n} having exactly k cycles.  Geoffrey Critzer, May 08 2012
Bessel numbers of the second kind. For relations to the Hermite polynomials and the Catalan (A033184 and A009766) and Fibonacci (A011973, A098925, and A092865) matrices, see Yang and Qiao.  Tom Copeland, Dec 18 2013.


LINKS

Table of n, a(n) for n=0..79.
P. Bala, Generalized Dobinski formulas
T. Copeland, Infinitesimal Generators, the Pascal Pyramid, and the Witt and Virasoro Algebras
H. Han, S. Seo, Combinatorial proofs of inverse relations and logconcavity for Bessel numbers, Eur. J. Combinat. 29 (7) (2008) 15441554. [From R. J. Mathar, Mar 20 2009]
S. Yang and Z. Qiao, The Bessel Numbers and Bessel Matrices, Journal of Mathematical Research & Exposition, July, 2011, Vol. 31, No. 4, pp. 627636. [From Tom Copeland, Dec 18 2013]


FORMULA

Number triangle T(n,k)=k!*C(n,k)/((2kn)!*2^(nk))
T(n,k) = A001498(k,nk).  Michael Somos, Oct 03 2006
E.g.f.: exp(y(x+x^2/2)).  Geoffrey Critzer, May 08 2012
Triangle equals the matrix product A008275*A039755. Equivalently, the nth row polynomial R(n,x) is given by the Type B Dobinski formula R(n,x) = exp(x/2)*sum {k = 0..inf} P(n,2*k+1)*(x/2)^k/k!, where P(n,x) = x*(x1)*...*(xn+1) denotes the falling factorial polynomial. Cf. A113278.  Peter Bala, Jun 23 2014


EXAMPLE

Triangle begins
1,
0, 1,
0, 1, 1,
0, 0, 3, 1,
0, 0, 3, 6, 1,
0, 0, 0, 15, 10, 1,
0, 0, 0, 15, 45, 15, 1,
0, 0, 0, 0, 105, 105, 21, 1


MATHEMATICA

t[n_, k_] := k!*Binomial[n, k]/((2 k  n)!*2^(n  k)); Table[ t[n, k], {n, 0, 11}, {k, 0, n}] // Flatten


PROG

(PARI) {T(n, k)=if(2*k<nk>n, 0, n!/(2*kn)!/(nk)!*2^(kn))} /* Michael Somos, Oct 03 2006 */


CROSSREFS

Cf. A001497, A049403, A111924. A008275, A039755, A113278.
Sequence in context: A170846 A085604 A144357 * A054548 A059202 A244963
Adjacent sequences: A122845 A122846 A122847 * A122849 A122850 A122851


KEYWORD

easy,nonn,tabl


AUTHOR

Paul Barry, Sep 14 2006


STATUS

approved



