  The t-t' Hubbard model for the Fermi level near the Van Hove singularity is
considered within the renormalization group and parquet approaches. The
interplay of ferromagnetic, antiferromagnetic, and superconducting channels is
investigated, and the phase diagram of the model is constructed. In comparison
with previous approaches, the account of ferromagnetic fluctuations suppresses
superconducting pairing and, vice versa, the influence of the Cooper channel
decreases the Curie temperature, so that the Stoner criterion is inapplicable
even qualitatively.
