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A105795 Shallow diagonal sums of the triangle k!*Stirling2(n,k): a(n) = Sum_{k=0..floor(n/2)} T(n-k,k) where T is A019538. 5
1, 0, 1, 1, 3, 7, 21, 67, 237, 907, 3741, 16507, 77517, 385627, 2024301, 11174587, 64673997, 391392667, 2470864941, 16237279867, 110858862477, 784987938907, 5755734591981, 43636725010747, 341615028340557, 2758165832945947, 22940755633301421, 196354180631212027 (list; graph; refs; listen; history; text; internal format)
OFFSET

0,5

COMMENTS

From Gus Wiseman, Jan 08 2019: (Start)

Also the number of set partitions of {1,...,n} into blocks of sizes > 1 whose minima form an initial interval of positive integers. For example, the a(5) = 7 set partitions are:

  {{1,2,3,4,5}}

  {{1,3},{2,4,5}}

  {{1,4},{2,3,5}}

  {{1,5},{2,3,4}}

  {{1,3,4},{2,5}}

  {{1,3,5},{2,4}}

  {{1,4,5},{2,3}}

Also the number of ordered set partitions of {1,...,n-k} of length k, for any 0 <= k <= n. For example, the a(5) = 7 ordered set partitions are:

  {{1,2,3,4}}

  {{1},{2,3}}

  {{2},{1,3}}

  {{3},{1,2}}

  {{1,2},{3}}

  {{1,3},{2}}

  {{2,3},{1}}

(End)

LINKS

Alois P. Heinz, Table of n, a(n) for n = 0..300

FORMULA

a(n) = sum{k=0..floor(n/2), sum{(-1)^i*binomial(k, i)*(k-i)^(n-k)}}.

E.g.f.: Sum_{n>=0} Integral^n (exp(x) - 1)^n dx^n, where integral^n F(x) dx^n is the n-th integration of F(x) with no constant of integration. - Paul D. Hanna, Dec 28 2013

Formal o.g.f.: 1/(1 + x)*sum {n >= 0} 1/(1 - n*x)*(x/(1 + x))^n = 1 + x^2 + x^3 + 3*x^4 + 7*x^5 + .... Cf. A229046. - Peter Bala, Jul 09 2014

EXAMPLE

a(8) = 1!*Stirling2(7,1) + 2!*Stirling2(6,2) + 3!*Stirling2(5,3) + 4!*Stirling2(4,4) = 1 + 62 + 150 + 24 = 237. - Peter Bala, Jul 09 2014

MAPLE

a:= n-> add(Stirling2(n-k, k)*k!, k=0..n/2):

seq(a(n), n=0..30);  # Alois P. Heinz, Jul 09 2014

MATHEMATICA

Table[Sum[StirlingS2[n-k, k]*k!, {k, 0, n/2}], {n, 0, 20}] (* Vaclav Kotesovec, Jul 16 2014 *)

sps[{}]:={{}}; sps[set:{i_, ___}]:=Join@@Function[s, Prepend[#, s]&/@sps[Complement[set, s]]]/@Cases[Subsets[set], {i, ___}];

Table[Sum[Length[Join@@Permutations/@Select[sps[Range[n-k]], Length[#]==k&]], {k, 0, n}], {n, 0, 10}] (* Gus Wiseman, Jan 08 2019 *)

PROG

(PARI) /* From Paul Barry's formula: */

{a(n)=sum(k=0, floor(n/2), sum(i=0, k, (-1)^i*binomial(k, i)*(k-i)^(n-k)))} \\ Paul D. Hanna, Dec 28 2013

for(n=0, 30, print1(a(n), ", "))

(PARI) /* From e.g.f. series involving iterated integration: */

{INTEGRATE(n, F)=local(G=F); for(i=1, n, G=intformal(G)); G}

{a(n)=local(A=1+x); A=1+sum(k=1, n, INTEGRATE(k, (exp(x+x*O(x^n))-1)^k )); n!*polcoeff(A, n)} \\ Paul D. Hanna, Dec 28 2013

CROSSREFS

Cf. A000110, A000296, A000670, A003101, A008277, A019538, A026898, A287215.

Sequence in context: A148677 A240506 A037127 * A322459 A244174 A148678

Adjacent sequences:  A105792 A105793 A105794 * A105796 A105797 A105798

KEYWORD

easy,nonn

AUTHOR

Paul Barry, Apr 20 2005

STATUS

approved

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Last modified May 30 15:21 EDT 2020. Contains 334726 sequences. (Running on oeis4.)