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2d
comment Angle between two vectors?
@Pedro77: However, my answer is correct formula for any two vectors in any $\mathbb{C}^n$. Note that the cross product of two vectors doesn't even make sense unless they are in $\mathbb{R}^3$ (Wikipedia).
2d
comment Angle between two vectors?
@Pedro77: No, it's not wrong. For two vectors $\mathbf{a},\mathbf{b}\in\mathbb{R}^3$, it's true that $$\tan^{-1}\left(\frac{\|\mathbf{a} \times \mathbf{b}\|}{\mathbf{a} \cdot\mathbf{b}}\right) =\tan^{-1}\left( \frac{\|\mathbf{a}\|\|\mathbf{b}\| \sin(\theta)}{\|\mathbf{a}\| \|\mathbf{b} \|\cos(\theta)}\right)= \tan^{-1} \left(\frac{\sin(\theta)}{\cos(\theta)}\right)= \tan^{-1}(\tan(\theta))=\theta$$ As was explained in the answer you linked to, that approach is chosen for numerical stability, which is a computational concern. Two formulas can both be correct but have different stability.
2d
answered If $f'(x)\rightarrow \infty$ then $f(x)\rightarrow\infty$?
2d
comment Conjecture: Partitioning $\Bbb N$ into parts that sum to $13^i$
Why not write the far clearer sentence "there exists a partition $\mathbb{N}=\bigcup A_i$ for which the sum of the elements of $A_i$ is $13^i$"?
Apr
27
revised Primary decomposition in a Dedekind Domain
added 5 characters in body
Apr
27
comment Why is the set of all half-lines in the real numbers not a topological space?
@BrahadeeshS.: The axiom you're thinking of is that any union of open sets is open.
Apr
27
answered Why is the set of all half-lines in the real numbers not a topological space?
Apr
25
awarded  Enlightened
Apr
23
revised On the degree of cyclotomic fields extension.
added 446 characters in body
Apr
23
answered On the degree of cyclotomic fields extension.
Apr
23
comment Is there a category in which, between any two objects, there is a unique morphism?
+1, agreed, not sure why you got a downvote.
Apr
23
revised Is there a category in which, between any two objects, there is a unique morphism?
edited title
Apr
23
revised Is there a category in which, between any two objects, there is a unique morphism?
added 237 characters in body
Apr
23
revised Is there a category in which, between any two objects, there is a unique morphism?
deleted 16 characters in body
Apr
23
answered Is there a category in which, between any two objects, there is a unique morphism?
Apr
22
revised Why does rotation by a quaternion require multiplying two times?
added 8 characters in body; edited title
Apr
22
reviewed Edit inverse of a root of f in an extension field K.
Apr
22
revised inverse of a root of f in an extension field K.
just notation
Apr
22
revised Which cyclotomic fields are different?
added 148 characters in body
Apr
22
answered Which cyclotomic fields are different?