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Published online by Cambridge University Press: 17 April 2009
Any linear operator A on 2 is shown to have two real quadratic form on S1 associated with it. They represent the expansion and rotation of the map and the eigenvalues of A can be described in a geometrically intrinsic way in therms of the eigenvalues of these two quadratic forms via the formula .
A number of theorems concerning these quadṙatic forms are presented.