A105484 Number of partitions of {1...n} containing 2 strings of 3 consecutive integers, where each string is counted within a block and a string of more than 3 consecutive integers are counted three at a time.

1, 2, 9, 38, 177, 882, 4711, 26795, 161583, 1028992, 6896067, 48487476, 356703531, 2738868784, 21901044795, 182022288438, 1569519971934, 14017732109520, 129480496353104, 1235228480628932, 12154988981496309, 123229919746398894, 1285758785855488107

Offset

4,2

Links

Table of n, a(n) for n=4..26.

Augustine O. Munagi, Set Partitions with Successions and Separations, Int. J. Math and Math. Sc., 2005:3 (2005), 451-463.

Formula

a(n) = Sum_{k=1..n} c(n, k, 2), where c(n, k, 2) is the case r =2 of c(n, k, r) given by c(n, k, r)=c(n-1, k-1, r)+(k-1)c(n-1, k, r)+c(n-2, k-1, r)+(k-1)c(n-2, k, r)+c(n-1, k, r-1)-c(n-2, k-1, r-1)-(k-1)c(n-2, k, r-1), r=0, 1, .., n-k-1, k=1, 2, .., n-2r, c(n, k, 0) = Sum_{j= 0..floor(n/2)} binomial(n-j, j)*S2(n-j-1, k-1).

Example

a(6)=9 because the partitions of {1,...,6} with 2 strings of 3 consecutive integers are 12346/5, 13456/2, 16/2345, 1234/56, 123/456, 12/3456, 1234/5/6, 1/2345/6, 1/2/3456.

Maple

c := proc(n, k, r) option remember ; local j ; if r =0 then add(binomial(n-j, j)*combinat[stirling2](n-j-1, k-1), j=0..floor(n/2)) ; else if r <0 or r > n-k-1 then RETURN(0) fi ; if n <1 then RETURN(0) fi ; if k <1 then RETURN(0) fi ; RETURN( c(n-1, k-1, r)+(k-1)*c(n-1, k, r)+c(n-2, k-1, r)+(k-1)*c(n-2, k, r) +c(n-1, k, r-1)-c(n-2, k-1, r-1)-(k-1)*c(n-2, k, r-1) ) ; fi ; end: A105484 := proc(n) local k ; add(c(n, k, 2), k=1..n) ; end: for n from 4 to 27 do printf("%d, ", A105484(n)) ; od ; # R. J. Mathar, Feb 20 2007

Mathematica

S2[_, -1] = 0;
S2[n_, k_] = StirlingS2[n, k];
c[n_, k_, r_] := c[n, k, r] = Which[r == 0, Sum[Binomial[n - j, j]*S2[n - j - 1, k - 1], {j, 0, Floor[n/2]}], r < 0 || r > n - k - 1, 0, n < 1, 0, k < 1, 0, True, c[n - 1, k - 1, r] + (k - 1)*c[n - 1, k, r] + c[n - 2, k - 1, r] + (k - 1)*c[n - 2, k, r] + c[n - 1, k, r - 1] - c[n - 2, k - 1, r - 1] - (k - 1)*c[n - 2, k, r - 1]];
A105484[n_] := Sum[c[n, k, 2], {k, 1, n}];
Table[A105484[n], {n, 4, 27}] (* Jean-François Alcover, May 10 2023, after R. J. Mathar *)

Crossrefs

Cf. A105483, A105485, A105488, A105492.

Sequence in context: A357547 A151004 A151005 * A151006 A151007 A151008

Adjacent sequences: A105481 A105482 A105483 * A105485 A105486 A105487

Keyword

nonn

Author

Augustine O. Munagi, Apr 10 2005

Extensions

More terms from R. J. Mathar, Feb 20 2007

Status

approved