A method for classification of red, blue and green galaxies using fuzzy
  set theory
  Red and blue galaxies are traditionally classified using some specific cuts
in colour or other galaxy properties, which are supported by empirical
arguments. The vagueness associated with such cuts are likely to introduce a
significant contamination in these samples. Fuzzy sets are vague boundary sets
which can efficiently capture the classification uncertainty in the absence of
any precise boundary. We propose a method for classification of galaxies
according to their colours using fuzzy set theory. We use data from the SDSS to
construct a fuzzy set for red galaxies with its members having different
degrees of `redness'. We show that the fuzzy sets for the blue and green
galaxies can be obtained from it using different fuzzy operations. We also
explore the possibility of using fuzzy relation to study the relationship
between different galaxy properties and discuss its strengths and limitations.
