Springel, Volker Cosmology, MPI for Astrophysics, Max Planck Society;
Hopkins, P. F., Hernquist, L., Cox, T. J., Di Matteo, T., Robertson, B., & Springel, V. (2005). Luminosity-dependent quasar lifetimes: reconciling the optical and X-ray quasar luminosity functions. The Astrophysical Journal, 632(1 Part 1), 81-91. doi:10.1086/432755.