67127
Conversely , singly even-dimensional manifolds have a " skew " - symmetric nondegenerate bilinear form on their middle dimension ; if one defines a quadratic refinement of this to a quadratic form ( as on a framed manifold ) , one obtains the Arf invariant as a mod 2 invariant .
 Arf
Please list all entity words in the text that fit the category. Output format is "word1, word2". 
Option: location, person, organization 
Text: {0} 
Answer:
 Arf