A343649 Triangle read by rows, 1 <= k <= n: T(n,k) is the number of (unlabeled) connected graphs with n nodes and throttling number k.

1, 0, 1, 0, 0, 2, 0, 0, 2, 4, 0, 0, 0, 13, 8, 0, 0, 0, 13, 83, 16, 0, 0, 0, 0, 308, 513, 32, 0, 0, 0, 0, 296, 7460, 3297, 64, 0, 0, 0, 0, 68, 38630, 200207, 22047, 128, 0, 0, 0, 0, 0, 65211, 4788564, 6709094, 153446, 256

Offset

1,6

Comments

The throttling number of a graph can be defined as follows. Start with a blue/white coloring of the nodes. At each step, all white nodes, which are currently the unique white neighbor of a blue node, are colored blue. The throttling number is the minimum, over all possible initial colorings, of the sum of the number of blue nodes in the initial coloring and the number of steps required to color all nodes blue.

A connected graph with n nodes has throttling number n if and only if it does not contain any 4-path, 4-cycle, or bowtie graph as an induced subgraph (Carlson and Kritschgau 2021, Theorem 4.2). Apparently, all these graphs can be constructed in the following way (for n >= 2). Let the nodes be 1, ..., n and choose a subset of special nodes. We require that n is a special node and that 1 is not, so there are 2^(n-2) possible choices for the set of special nodes. For i < j, let there be an edge between i and j if and only if j is a special node. These graphs do not contain any of the forbidden induced subgraphs, and different sets of special nodes lead to nonisomorphic graphs. Consequently, T(n,n) >= 2^(n-2). Apparently, equality holds, so that there are no other such graphs.

Links

Table of n, a(n) for n=1..55.

Steve Butler and Michael Young, Throttling zero forcing propagation speed on graphs, Australasian Journal of Combinatorics 57 (2013), 65-71.

Joshua Carlson and Juergen Kritschgau, Various characterizations of throttling numbers, arXiv:1909.07952 [math.CO], 2021.

Formula

T(n,k) = 0 for k < A000267(n-1), and T(n,A000267(n-1)) > 0 because the n-path has throttling number A000267(n-1) (Butler and Young 2013).

Example

Triangle begins:
   n\k 1  2  3   4    5      6        7        8       9   10
  -----------------------------------------------------------
   1:  1
   2:  0  1
   3:  0  0  2
   4:  0  0  2   4
   5:  0  0  0  13    8
   6:  0  0  0  13   83     16
   7:  0  0  0   0  308    513       32
   8:  0  0  0   0  296   7460     3297       64
   9:  0  0  0   0   68  38630   200207    22047     128
  10:  0  0  0   0    0  65211  4788564  6709094  153446  256

Crossrefs

Row sums: A001349.

Cf. A000267, A343648.

Sequence in context: A279360 A134312 A329790 * A195581 A020474 A135589

Adjacent sequences: A343646 A343647 A343648 * A343650 A343651 A343652

Keyword

nonn,tabl

Author

Pontus von Brömssen, Apr 24 2021

Status

approved