To cite SPO in publications use:

Bartz-Beielstein T, Lasarczyk C, Preuss M (2005). “Sequential Parameter Optimization.” In Proceedings Congress on Evolutionary Computation 2005 (CEC'05), 1553. http://www.spotseven.de/wp-content/papercite-data/pdf/blp05.pdf.

Corresponding BibTeX entry:

  @InProceedings{,
    title = {Sequential Parameter Optimization},
    author = {Thomas Bartz-Beielstein and Christian Lasarczyk and Mike
      Preuss},
    booktitle = {Proceedings Congress on Evolutionary Computation 2005
      (CEC'05)},
    year = {2005},
    address = {Edinburgh, Scotland},
    pages = {1553},
    url =
      {http://www.spotseven.de/wp-content/papercite-data/pdf/blp05.pdf},
  }