To cite GNU Octave in publications use:

  Ramsay, J., Hooker, G., & Graves, S. (2009). 
  Functional Data Analysis with R and MATLAB. 
  New York, NY: Springer New York. 
  DOI: 10.1007/978-0-387-98185-7
 
A BibTeX entry for LaTeX users is:

  @book{,
    address   = {New York, NY},
    author    = {Ramsay, James and Hooker, Giles and Graves, Spencer},
    doi       = {10.1007/978-0-387-98185-7},
    isbn      = {978-0-387-98184-0},
    publisher = {Springer New York},
    title     = {{Functional Data Analysis with R and MATLAB}},
    url       = {www.functionaldata.org},
    year      = {2009}
  }
