# extremal combinatorics

## Sidorenko's Conjecture ★★★

Author(s): Sidorenko

**Conjecture**For any bipartite graph and graph , the number of homomorphisms from to is at least .

Keywords: density problems; extremal combinatorics; homomorphism

## Turán Problem for $10$-Cycles in the Hypercube ★★

Author(s): Erdos

**Problem**Bound the extremal number of in the hypercube.

Keywords: cycles; extremal combinatorics; hypercube

## Extremal $4$-Neighbour Bootstrap Percolation in the Hypercube ★★

**Problem**Determine the smallest percolating set for the -neighbour bootstrap process in the hypercube.

Keywords: bootstrap percolation; extremal combinatorics; hypercube; percolation

## Saturated $k$-Sperner Systems of Minimum Size ★★

Author(s): Morrison; Noel; Scott

**Question**Does there exist a constant and a function such that if , then every saturated -Sperner system has cardinality at least ?

Keywords: antichain; extremal combinatorics; minimum saturation; saturation; Sperner system