Halo mass function with Planck Collaboration VI (2020) cosmological values (Ω_Λ ≈ 0.7, Ω_m + Ω_a ≈ 0.3) at z = 0.1 with a fixed relative ULA density of Ω_a/(Ω_cdm + Ω_a) = 0.5 and varying ULA mass (left) and a fixed ULA mass of m_a = 10⁻²⁵ eV and varying ULA abundance (right) compared to the ΛCDM HMF (black). Low-mass halo suppression becomes stronger with decreasing ULA mass and increasing ULA abundance. For higher ULA masses of m_a ≳ 10⁻²⁴ eV, the HMF becomes indistinguishable from the one in a ΛCDM cosmology, as is the case for Ω_a → 0.