  We propose a quantitative model for triplet superconductivity in Sr_2RuO_4
based on first principles calculations for the electronic structure and
magnetic susceptibility. The superconductivity is due to ferromagnetic spin
fluctuations, that are strong at small wave vectors. The calculated effective
mass renormalization, renormalized susceptibility, and superconducting critical
temperature are all in good agreement with experiment. The most stable
superconducting solution has order parameters of comparable magnitude on all
three sheets of the Fermi surface. We also propose an explanation of the
discrepancy between the Fermi surface topology observed in the photoemission
and in de Haas-van Alphen experiments.
