Conformal bootstrap: any CFT correlation function must satisfy crossing symmetry of the 4-point function: Σ_O λ²_O F_O(u,v) = 0.
This constrains operator dimensions Δ_O and OPE coefficients λ_O. The unitarity bound requires Δ ≥ (d−2)/2 for scalars.
By scanning (Δ_σ, Δ_ε) space and demanding the crossing equation has a valid solution, Rattazzi-Rychkov-Tonni-Vichi (2008) placed sharp bounds.
The 3D Ising CFT sits at the tip of the allowed region: Δ_σ = 0.5181489..., Δ_ε = 1.412625... (Kos-Poland-Simmons-Duffin 2016, to 8 significant figures).