Lemma 3.6. Assuming that:

$ψ : V \to ℝ$
$⟨ ψ, 1 ⟩ = 0$
let $(S, V ∖ S) = Fiedler (G, ψ)$

Then

Φ (S, V ∖ S) \leq \sqrt{2 q_{{\tilde{L}}_{G}} (ψ)} .