@article{etna_vol44_pp367-400,
  author  = {Yue Qiu and Martin B. van Gijzen and Jan-Willem van Wingerden and Michel Verhaegen and Cornelis Vuik},
  title   = {Efficient preconditioners for PDE-constrained optimization problem with a multilevel sequentially semiseparable matrix structure},
  journal = {Electron. Trans. Numer. Anal.},
  volume  = {44},
  year    = {2015},
  pages   = {367--400},
}