Before revision: The S-procedure was developed independently in a number of different contextsFrank Uhlig, A recurring theorem about pairs of quadratic forms and extensions: a survey, Linear Algebra and its Applications, Volume 25, 1979, Pages 219-237.
After revision: The S-procedure was developed independently in a number of different contextsFrank Uhlig, A recurring theorem about pairs of quadratic forms and extensions: a survey, Linear Algebra and its Applications, Volume 25, 1979,