کد مقاله | کد نشریه | سال انتشار | مقاله انگلیسی | نسخه تمام متن |
---|---|---|---|---|
6416354 | 1631129 | 2015 | 22 صفحه PDF | دانلود رایگان |
Let H=H+âHâ be an orthogonal decomposition of a Hilbert space, with E+, Eâ the corresponding projections. Let A be a selfadjoint operator in H which is codiagonal with respect to this decomposition (i.e. A(H+)âHâ and A(Hâ)âH+). We consider three maps which assign a selfadjoint projection to A:1.The graph map Î: Î(A)=projection onto the graph of A|H+.2.The exponential map of the Grassmann manifold P of H (the space of selfadjoint projections in H) at E+: expâ¡(A)=eiÏ2AE+eâiÏ2A.3.The map p, called here the Davis' map, based on a result by Chandler Davis, characterizing the selfadjoint contractions which are the difference of two projections.The ranges of these maps are studied and compared.Using Davis' map, one can solve the following operator matrix completion problem: given a contraction a:HââH+, complete the matrix(âa/2aâ/2â)to a projection P, in order that âPâE+â is minimal.
Journal: Linear Algebra and its Applications - Volume 466, 1 February 2015, Pages 307-328