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A060052 Triangle read by rows: T(n,k) gives number of r-bicoverings of an n-set with k blocks, n >= 2, k = 3..n+floor(n/2). 11
1, 1, 4, 0, 15, 25, 3, 0, 30, 222, 226, 40, 0, 30, 1230, 3670, 2706, 535, 15, 0, 0, 5040, 39900, 69450, 40405, 8141, 420, 0, 0, 15120, 345240, 1254960, 1498035, 722275, 142877, 9730, 105, 0, 0, 30240, 2492280, 18587520, 40701780, 36450820, 15031204, 2871240, 226828, 5040 (list; graph; refs; listen; history; text; internal format)
OFFSET

2,3

COMMENTS

A bicovering is r-bicovering if intersection of every two blocks contains at most one element.

REFERENCES

I. P. Goulden and D. M. Jackson, Combinatorial Enumeration, John Wiley and Sons, N.Y., 1983.

LINKS

Andrew Howroyd, Table of n, a(n) for n = 2..1802 (rows n=2..50)

Table

FORMULA

E.g.f.: A(x, y) = exp(-x-1/2*x^2*y)*Sum_{i>=0} (1+y)^binomial(i, 2)*x^i/i!.

T(n, k) = (n!/k!) * A276640(k, n). - David Pasino, Sep 22 2016

T(n,k) = 0 for n > binomial(k,2). - Andrew Howroyd, Jan 30 2020

EXAMPLE

Triangle starts:

[1],

[1, 4],

[0, 15, 25, 3],

[0, 30, 222, 226, 40],

[0, 30, 1230, 3670, 2706, 535, 15],

[0, 0, 5040, 39900, 69450, 40405, 8141, 420],

[0, 0, 15120, 345240, 1254960, 1498035, 722275, 142877, 9730, 105],

[0, 0, 30240, 2492280, 18587520, 40701780, 36450820, 15031204, 2871240, 226828, 5040],

...

PROG

(PARI) \\ returns k-th column as vector.

C(k)=if(k<3, [], Vecrev(serlaplace(polcoef(exp(-x-1/2*x^2*y + O(x*x^k))*sum(i=0, 3*k\2, (1+y)^binomial(i, 2)*x^i/i!), k))/y)) \\ Andrew Howroyd, Jan 30 2020

(PARI)

T(n)={my(m=(3*n\2), y='y + O('y^(n+1))); my(g=exp(-x-1/2*x^2*y + O(x*x^m))*sum(k=0, m, (1+y)^binomial(k, 2)*x^k/k!)); Mat([Col(serlaplace(p), -n) | p<-Vec(g)[2..m+1]])}

{ my(A=T(8)); for(n=2, matsize(A)[1], print(A[n, 3..3*n\2])) } \\ Andrew Howroyd, Jan 30 2020

CROSSREFS

Row sums are A060053.

Column sums are A060051.

Cf. A059443, A059530, A060092, A060487, A060492, A276640, A331039.

Sequence in context: A141150 A081162 A095367 * A059065 A170771 A170772

Adjacent sequences:  A060049 A060050 A060051 * A060053 A060054 A060055

KEYWORD

easy,nonn,tabf

AUTHOR

Vladeta Jovovic, Feb 15 2001

EXTENSIONS

Zeros inserted into data by Andrew Howroyd, Jan 30 2020

STATUS

approved

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Last modified June 24 20:41 EDT 2021. Contains 345425 sequences. (Running on oeis4.)