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Saturation in the Hypercube and Bootstrap Percolation

Part of: Graph theory Extremal combinatorics

Published online by Cambridge University Press:  31 March 2016

NATASHA MORRISON ,
JONATHAN A. NOEL  and
ALEX SCOTT
NATASHA MORRISON
Affiliation:
Mathematical Institute, University of Oxford, Andrew Wiles Building, Radcliffe Observatory Quarter, Woodstock Road, Oxford OX2 6GG, UK (e-mail: morrison@maths.ox.ac.uk, noel@maths.ox.ac.uk, scott@maths.ox.ac.uk)
JONATHAN A. NOEL
Affiliation:
Mathematical Institute, University of Oxford, Andrew Wiles Building, Radcliffe Observatory Quarter, Woodstock Road, Oxford OX2 6GG, UK (e-mail: morrison@maths.ox.ac.uk, noel@maths.ox.ac.uk, scott@maths.ox.ac.uk)
ALEX SCOTT
Affiliation:
Mathematical Institute, University of Oxford, Andrew Wiles Building, Radcliffe Observatory Quarter, Woodstock Road, Oxford OX2 6GG, UK (e-mail: morrison@maths.ox.ac.uk, noel@maths.ox.ac.uk, scott@maths.ox.ac.uk)
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Abstract

Let Qd denote the hypercube of dimension d. Given d ⩾ m, a spanning subgraph G of Qd is said to be (Qd, Qm)-saturated if it does not contain Qm as a subgraph but adding any edge of E(Qd)\E(G) creates a copy of Qm in G. Answering a question of Johnson and Pinto [27], we show that for every fixed m ⩾ 2 the minimum number of edges in a (Qd, Qm)-saturated graph is Θ(2d).

We also study weak saturation, which is a form of bootstrap percolation. A spanning subgraph of Qd is said to be weakly (Qd, Qm)-saturated if the edges of E(Qd)\E(G) can be added to G one at a time so that each added edge creates a new copy of Qm. Answering another question of Johnson and Pinto [27], we determine the minimum number of edges in a weakly (Qd, Qm)-saturated graph for all d ⩾ m ⩾ 1. More generally, we determine the minimum number of edges in a subgraph of the d-dimensional grid Pkd which is weakly saturated with respect to ‘axis aligned’ copies of a smaller grid Prm. We also study weak saturation of cycles in the grid.

MSC classification

Primary: 05C35: Extremal problems
Secondary: 05C50: Graphs and linear algebra (matrices, eigenvalues, etc.) 05D99: None of the above, but in this section
Type
Paper
Information
Combinatorics, Probability and Computing , Volume 26 , Issue 1 , January 2017 , pp. 78 - 98
Copyright
Copyright © Cambridge University Press 2016 

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