Geoff Robinson's user avatar
Geoff Robinson's user avatar
Geoff Robinson's user avatar
Geoff Robinson
  • Member for 12 years, 9 months
  • Last seen this week
  • United Kingdom
96 votes

Is it true that $0.999999999\ldots=1$?

63 votes

The identity cannot be a commutator in a Banach algebra?

56 votes
Accepted

Why is the complex number $z=a+bi$ equivalent to the matrix form $\left(\begin{smallmatrix}a &-b\\b&a\end{smallmatrix}\right)$

31 votes
Accepted

How to prove that $\mathbb{Q}[\sqrt{p_1}, \sqrt{p_2}, \ldots,\sqrt{p_n} ] = \mathbb{Q}[\sqrt{p_1}+ \sqrt{p_2}+\cdots + \sqrt{p_n}]$, for $p_i$ prime?

26 votes
Accepted

proper subgroups of finite p-groups are properly contained in the normalizer

24 votes

How is a group made up of simple groups?

22 votes
Accepted

Upper bounds on the size of $\operatorname{Aut}(G)$

20 votes
Accepted

subgroup generated by two subgroups

20 votes

Matrices which are both unitary and Hermitian

19 votes

If $\sum_{n=1}^{\infty} a_n^{2}$ converges, then so does $\sum_{n=1}^{\infty} \frac {a_n}{n}$

18 votes

Understanding of the theorem that all norms are equivalent in finite dimensional vector spaces

18 votes
Accepted

Non surjectivity of the exponential map to GL(2,R)

15 votes
Accepted

Easy ways to find partial fraction representation? (via a concrete example)

15 votes

If $A,B$ symmetric positive semidefinite, show tr$(AB) \geq 0$

14 votes
Accepted

How to prove $\sum_{n=0}^\infty \frac1{n!}=e\ $?

14 votes

Spectrum of a nilpotent operator

13 votes
Accepted

If $H\unlhd G$ with $(|H|,[G:H])=1$ then $H$ is the unique such subgroup in $G$.

12 votes
Accepted

Are two subgroups that contain a common element conjugate iff they are conjugate under the normalizer?

12 votes
Accepted

If $|H|$ and $[G:K]$ are relatively prime, then $H \leq K$

12 votes

Outer Automorphisms of $S_n$

12 votes
Accepted

If $C$ commutes with certain matrices $A$ and $B$, why is $C$ a scalar multiple of the identity?

12 votes
Accepted

Any group of order $2p$ has a subgroup of order $p~$($p$ being a prime)

12 votes

Known bounds for the number of groups of a given order.

12 votes

How can one convert rational matrices into integer ones?

12 votes

Show that two matrices with the same eigenvalues are similar

11 votes

Example where a finite group $G$ of order $n$ has no subgroup of order $m$

11 votes

Does a Conjugacy Class always contain an element and its inverse?

11 votes

Embedding $GL_2(\mathbb{F}_l)$ into $GL_2(\mathbb{C})$

11 votes

Proof that no permutation can be expressed both as the product of an even number of transpositions and as a product of an odd number of transpositions

11 votes

Find the number of subgroups of order $p$

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