A projection which has $||E\alpha|| \le ||\alpha||$ is an orthogonal projection
93165
A projection satisfying $\| Px \| \leq \|x\|$ for all $x$ is an orthogonal projection
113326
Proving orthogonality of vectors in a complex vector space
42606
Orthogonality for Binomial Coefficients
115272
Showing that projections $\mathbb{R}^2 \to \mathbb{R}$ are not closed
146812
Explicit construction for unitary extensions of completely positive and trace preserving (CPTP) maps?
146099
Why is this true: The only orthogonal projection that is also unitary from $\Bbb C^n$ to $\Bbb C^n$ is the identity
81112
Proving that the dual of the $\mathcal{l}_p$ norm is the $\mathcal{l}_q$ norm.
3870
A variation of Borel Cantelli Lemma
81775
Prove $a^\alpha b^{1-\alpha} \le \alpha a + (1 - \alpha)b, \; a,b > 0,\; 0 < \alpha < 1$
92497
Isometric Embedding of a separable Banach Space into $\ell^{\infty}$
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 $PH\cap QH$
70415
Comparison between the $\ell_1$ and $\ell_2$ norms
112231
Showing that a one-to-one linear transformation maps a linearly independent set onto a linearly independent set
97454
Find the value of $\alpha-\beta$ if $g(x,y)=\alpha x+\beta y$ is a Hahn-Banach extension of $f$
83970
Proof of an elementary property of Projection Operators
17031
Idempotent linear operators are projections
90757
$T$ self-adjoint on Hilbert space $H$, $T^n=T^{n+1}$ will $T$ be an orthogonal projection operator?