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A202830 E.g.f: exp(2*x + 3*x^2/2). 1
1, 2, 7, 26, 115, 542, 2809, 15374, 89737, 548450, 3519799, 23493098, 163139563, 1172030654, 8706504265, 66638295998, 525069283921, 4248776775746, 35276087031463, 299986119953210, 2610709200699811, 23220585598592222, 210915850841272537, 1954390351189631726 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,2

COMMENTS

From Emanuele Munarini, May 16 2014: (Start)

a(n) is the number of partitions of an n-set where each block consists of one or two elements, and each block is endowed with a permutation or a linear order. For instance, for n = 2, we have the following partitions of the set {1,2}:

(1)(2), (1)[2], [1](2), [1][2], (12), [12], [21]

where parenthesis denote blocks consisting of cycles, and square brackets denote blocks consisting of linear orders. (End)

LINKS

Table of n, a(n) for n=0..23.

FORMULA

a(n) = Sum_{k=0..[n/2]} 2^(n-3*k)*3^k * n!/((n-2*k)!*k!).

O.g.f.: 1/(1-2*x - 3*x^2/(1-2*x - 6*x^2/(1-2*x - 9*x^2/(1-2*x - 12*x^2/(1-2*x -...))))), (continued fraction).

E.g.f.: exp((4*x+3*x^2)/2) = G(0) ; G(k)= 1+(4*x+3*x^2)/(4*k+2-(4*x+3*x^2)*(4*k+2)/(4*x+3*x^2+4*(k+1)/G(k+1))) ; (continued fraction). - Sergei N. Gladkovskii, Dec 28 2011

a(n) = 2*a(n-1) + 3*(n-1)*a(n-2), a(0)=1, a(1)=2. - Sergei N. Gladkovskii, Jul 29 2012

a(n) ~ exp(2/3*sqrt(3*n)-n/2-1/3)*3^(n/2)*n^(n/2)/sqrt(2)*(1+11/54*sqrt(3)/sqrt(n)). - Vaclav Kotesovec, Oct 20 2012

a(n) = sum(s(n,k)*(-1)^(n-k)*3^n/2^(n-k)*B(k,2/3),k,0,n), where the s(n,k) are the (signless) Stirling numbers of the first kind and the B(n,x) = sum(S(n,k)*x^k,k=0..n) are the Stirling polynomials (or exponential polynomials), where the S(n,k) are the Stirling numbers of the second kind. - Emanuele Munarini, May 15 2014

a(n) = I^(1-n)*2^((1+n)/2)*3^((n-1)/2)*U((1-n)/2, 3/2, -2/3), U Kummer's hypergeometric function of the second kind. - Peter Luschny, May 15 2014

EXAMPLE

E.g.f.: A(x) = 1 + 2*x + 7*x^2/2! + 26*x^3/3! + 115*x^4/4! + 542*x^5/5! +...

MAPLE

a := n -> I^(1-n)*2^((1+n)/2)*3^((n-1)/2)*KummerU((1-n)/2, 3/2, -2/3);

seq(round(evalf(a(n), 32)), n=0..23); # Peter Luschny, May 15 2014

MATHEMATICA

CoefficientList[Series[E^(2*x+3*x^2/2), {x, 0, 20}], x]* Range[0, 20]! (* Vaclav Kotesovec, Oct 20 2012 *)

a[n_] := Sum[StirlingS1[n, k]*3^n/2^(n - k) BellB[k, 2/3], {k, 0, n}]; Table[a[n], {n, 0, 12}] (* Emanuele Munarini, May 15 2014 *)

PROG

(PARI) {a(n)=n!*polcoeff(exp(2*x+3*x^2/2+x*O(x^n)), n)}

(PARI) {a(n)=sum(k=0, n\2, 2^(n-3*k)*3^k*n!/((n-2*k)!*k!))}

(PARI) /* O.g.f. as a continued fraction: */

{a(n)=local(CF=1+2*x+x*O(x^n)); for(k=1, n-1, CF=1/(1-2*x-3*(n-k)*x^2*CF)); polcoeff(CF, n)}

(Maxima) B(n, x) := sum(stirling2(n, k)*x^k, k, 0, n);

a(n) := sum(stirling1(n, k)*3^n/2^(n-k)*B(k, 2/3), k, 0, n);

makelist(a(n), n, 0, 40); /* Emanuele Munarini, May 15 2014 */

CROSSREFS

Cf. A202829.

Sequence in context: A150589 A007168 A150590 * A006373 A030429 A167551

Adjacent sequences:  A202827 A202828 A202829 * A202831 A202832 A202833

KEYWORD

nonn

AUTHOR

Paul D. Hanna, Dec 25 2011

STATUS

approved

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Last modified April 1 14:05 EDT 2020. Contains 333159 sequences. (Running on oeis4.)