Is $\|T(x)\|≦\|x\|$, $\forall x\in V$ sufficient condition for $T$ to be an orthogonal projection?
93165
A projection satisfying $\| Px \| \leq \|x\|$ for all $x$ is an orthogonal projection
57609
Hahn-Banach proof by extension of basis
3038
For positive invertible operators $C\leq T$ on a Hilbert space, does it follow that $T^{-1}\leq C^{-1}$?
52791
Invertibility of an operator $T$ if $\|I-T\|<1$
90757
$T$ self-adjoint on Hilbert space $H$, $T^n=T^{n+1}$ will $T$ be an orthogonal projection operator?
112362
fixed point problem for orthogonal transformation
36567
$A^{T}A$ positive definite then A is invertible?
71350
Let $H$ be a Hilbert space with projections $P,Q$, then $\lim_{n\to\infty}(PQ)^nx= Rx$ for all $x\in H$, where $R$ is the projection to $PH\cap QH$
141286
When is $\mathbf{X}^{T}\mathbf{X}+\lambda\mathbf{I}$ invertible?
59531
Do $T$-invariant subspaces necessarily have a $T$-invariant complement?
73406
Prove that $E_{\lambda}(T)=E_{\lambda^{-1}}(T^{-1})$ if $T$ is an invertible linear operator on a finite dimensional vector space $V$
17031
Idempotent linear operators are projections
69474
Orthogonal projections with $\sum P_i =I$, proving that $i\ne j \Rightarrow  P_{j}P_{i}=0$
156922
Proving convergence iff projection converges in product topology
258
Existence of orthogonal coordinates on a Riemannian manifold
16692
a representation condition of Hilbert space
97392
If $e^{itx_n}$ converges for every $t\in\mathbb R$, then does $x_n$ converge?