给出关于投影算子的两个注记:首先,对由一组线性无关元决定的有限秩投影作出特征刻画;其次,对于Hilbert空间上两个正交投影算子P1和P2,证明算子列{(P1P2)^n}n=1^∞是逼近正交投影算子.
Give two notes on the projection : firstly, discribe the characteristics of the finite rank projection determined by a group of linearly independent elements ; secondly, let P1 and P2 be two orthogonal projections on Hilbert space, prove that the sequence of operators {(P1P2)^n}n=1^∞ is an approximate orthogonal projection.