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Jan
17
comment De Rham cohomology of formal groups
@Neil: that would be great, thank you!
Jan
17
awarded  Student
Jan
17
asked De Rham cohomology of formal groups
Jan
17
comment If Ramanujan's tau function has a prime power zero then $\ldots$
@Luis: Where in the proof of "if $\tau(p^n)=0$ then $\tau(p)=0$" (bottom of page 430 and top of page 431) is it assumed?
Jan
17
comment If Ramanujan's tau function has a prime power zero then $\ldots$
@Luis: if I'm not mistaken, the proof of theorem 2 consists in showing that if $\tau(p^n)=0$ then $\tau(p)=0$ which does answer your question.
Jan
17
answered If Ramanujan's tau function has a prime power zero then $\ldots$
Dec
31
comment The closures in $C^0(\mathbb{R}, \mathbb{R})$ of the set of integer valued polynomials, resp, of polynomials with integer coefficients
@Paul: Hi Paul! I wrote a survey of Ferguson's results which you can find at the very bottom of this page umpa.ens-lyon.fr/~lberger/publications.html (it's in French, sorry). The main results are that if $K$ is a compact subset of $R$ then $Z[X]$ is discrete in $C^0(K,R)$ if the capacity of $K$ is $\geq 1$ and otherwise there is a finite set $J(K) \subset K$ such that a function $f$ is a limit of elements of $Z[X]$ if and only if there exists some $P(X) \in Z[X]$ such that $f(a)=P(a)$ for all $a \in J(K)$. For example if $K=[0,1]$ then $J(K)=\{0,1\}$.
Dec
19
answered Induced representations and $(\varphi, \Gamma)-modules
Dec
15
comment Explicitly describing a two-dimensional reducible representation of G_{Q_p}
Although it's not very explicit, you can say that $H^1(G_{Q_p(\mu_{p^\infty})},Qp)$ is a $\Lambda$-module and by the inflation-restriction map you character is in the part on which $\Gamma$ acts by $\chi^j$. If you're wondering what $H^1(G_{Q_p(\mu_{p^\infty})},Qp)$ looks like, the theory of $(\varphi,\Gamma)$-modules may help...
Dec
9
awarded  Critic
Dec
7
answered Never appeared forthcoming papers
Nov
7
answered What is the p-adic valuation of a number?
Nov
4
comment What are the $p$-adic representations of $\hat{\mathbb{Z}}$ ?
@Kevin: yes I think so! Just to recap: the continuous image of a compact set is compact so the image has to land in a compact subgroup of $GL_n(Q_p)$ which we know is conjugate to a subgroup of $GL_n(Z_p)$. Conversely if $M = card(GL_n(F_p))$ and $f(1)$ is (say) in $GL_n(Z_p)$ then for every $k$, the image of $p^{k-1}M Z$ is in $1+p^k M_n(Z_p)$ so the map from $Z$ extends by uniform continuity. To relate this to Torsten's answer, it remains to check that a matrix is conjugate to an element of $GL_n(Z_p)$ iff its char poly has integral coeffts.
Nov
1
awarded  Fanatic
Oct
24
answered Can an etale (phi, Gamma) module be an extension of non-etale ones?
Oct
24
comment Can an etale (phi, Gamma) module be an extension of non-etale ones?
The last category is closed under extensions. What you mean is something different, isn't it?
Oct
22
comment Citing papers that are in a language that you do not read.
I don't think it's necessary to have read the whole paper in order to cite it, but you need to point to a precise result in the reference, and explain the translation between the result as it's stated, and the results as you use it. I find it annoying when I see stg like "by [6], we have..." when [6] is a book, especially when the author of the paper is unable to say where in the book the result is when you ask him.
Oct
11
comment When should a result be made into a paper?
@Kimball: Yes, Rota said that, see for instance: math.ohio-state.edu/~nevai/MYMATH/rota_ams_notices_01_97.html
Oct
10
awarded  Good Answer
Oct
9
awarded  Commentator