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A260878 Number of set partitions of {1, 2, ..., 2*n} with sizes in {[n, n], [2n]}. 9
2, 2, 4, 11, 36, 127, 463, 1717, 6436, 24311, 92379, 352717, 1352079, 5200301, 20058301, 77558761, 300540196, 1166803111, 4537567651, 17672631901, 68923264411, 269128937221, 1052049481861, 4116715363801, 16123801841551, 63205303218877, 247959266474053 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,1

COMMENTS

Third column in A260876.

LINKS

Alois P. Heinz, Table of n, a(n) for n = 0..1000

Hans Salié, Über die Koeffizienten der Blasiusschen Reihe, Math. Nachr. 1955, (14, 4-6), 241--248.

FORMULA

G.f.: (4*x^2 - 13*x + 3 + sqrt((1 - 4*x)*(x - 1)^2))/(2*(4*x - 1)*(x - 1)). - Alois P. Heinz, Aug 06 2015

a(n) = Binomial(2*n-1, n) + 1. - Vladimir Kruchinin, Feb 26 2017

The generating function G(x) satisfies the differential equation x^3 + 2*x = (4*x^4 - 9*x^3 + 6*x^2 - x)*diff(G(x), x) + (2*x^3 - 4*x^2 + 2*x)*G(x). - Peter Luschny, Feb 12 2019

From Peter Luschny, Aug 02 2019: (Start)

a(n) = ((4*n - 2)*a(n-1) - 3*n + 2)/n for n >= 2.

a(n) = (2*n)! * [x^(2*n)] exp(exp(x)*(1 - (Gamma(n,x)/Gamma(n)))) for n >= 2.

a(n) ~ 4^n/sqrt(4*Pi*n). More precise asymptotic estimates are:

1 + (4^n/sqrt(n*Pi)) * (1/2 - 1/(16*n) * (1 - 1/(16*n))), and

1 + 4^n*(2 - 2/N^2 + 21/N^4 - 671/N^6) / sqrt(2*N*Pi) with N = 8*n + 2.

Let b(n) = binomial(2*(n-1), n-1) + 1 = A323230(n) for n >= 0. Then by Salié:

p divides a(p+k) - b(k+1) if p is a prime > k and 0 <= k <= 4.

Conjecture: p divides a(p+5) - b(6) if p is a prime > b(6).

If p is a prime divisor of n then a(n) == a(n/p) (mod p) (by Salié, theorem 2).

(End)

EXAMPLE

The set partitions counted by a(3) = 11 are: {{1, 2, 3, 4, 5, 6}},

{{1, 2, 4}, {3, 5, 6}}, {{1, 2, 3}, {4, 5, 6}}, {{1, 3, 4}, {2, 5, 6}},

{{1, 3, 5}, {2, 4, 6}}, {{1, 4, 5}, {2, 3, 6}}, {{1, 5, 6}, {2, 3, 4}},

{{1, 4, 6}, {2, 3, 5}}, {{1, 3, 6}, {2, 4, 5}}, {{1, 2, 6}, {3, 4, 5}},

{{1, 2, 5}, {3, 4, 6}}.

MAPLE

a := proc(n) option remember;

if n < 2 then [2, 2][n+1] else ((4*n - 2)*a(n-1) - 3*n + 2)/n fi end:

seq(a(n), n=0..26); # Or:

egf := n -> exp(exp(x)*(1 - (GAMMA(n, x)/GAMMA(n)))):

a := n -> `if`(n<2, 2, (2*n)!*coeff(series(egf(n), x, 2*n+1), x, 2*n)):

seq(a(n), n=0..26); # Peter Luschny, Aug 02 2019

MATHEMATICA

Table[Binomial[2 n - 1, n] + 1, {n, 0, 26}] (* or *)

CoefficientList[Series[(4 x^2 - 13 x + 3 + Sqrt[(1 - 4 x) (x - 1)^2])/(2 (4 x - 1) (x - 1)), {x, 0, 26}], x] (* Michael De Vlieger, Feb 26 2017 *)

PROG

(Sage)

print([A260876(n, 2) for n in (0..30)])

(Sage) # Alternative:

def A260878():

    a, f, s, n = 2, 2, 1, 1

    yield a

    while True:

        yield a

        f += 4; s += 3; n += 1

        a = (f*a - s)/n

a = A260878()

print([next(a) for n in range(27)]) # Peter Luschny, Aug 02 2019

CROSSREFS

a(n) = A112849(n) for n >= 2. - Alois P. Heinz, Aug 06 2015

a(n) = A052473(n+2) - 1.

a(n) = A088218(n) + 1.

a(n) = (-1)^n*A110556(n) + 1.

a(n+1) - a(n) = A097613(n+1) for n > 0.

Cf. A260876, A001700.

Cf. A323230 (d=0), this sequence (d=1), A323229 (d=2).

Sequence in context: A153950 A153947 A298778 * A064880 A115011 A296688

Adjacent sequences:  A260875 A260876 A260877 * A260879 A260880 A260881

KEYWORD

nonn,easy

AUTHOR

Peter Luschny, Aug 02 2015

STATUS

approved

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Last modified October 26 06:11 EDT 2020. Contains 338027 sequences. (Running on oeis4.)