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bio website cs.ox.ac.uk/people/…
location Oxford, United Kingdom
age 50
visits member for 3 years, 4 months
seen 18 hours ago

Oct
29
awarded  Popular Question
Oct
11
comment How close to platonic can a non-platonic planar graph be?
You get quite a bit of constraints from Euler formula $v-e+f=2$: your graph is regular, i.e. $2e=vk$, and the $f-1$ faces are $\ell$-gonal, while the remaining face is $\ell'$-gonal. So this gives $2e=(f-1)\ell+\ell'$. Finally, don't forget topology, which forbids $\ell$ greater than 5...
Oct
2
awarded  Caucus
Sep
21
comment Does there exist a polar decomposition of matrices over finite fields?
Perhaps you should explain the motivation you have for such a decomposition.
Sep
19
comment Finding automorphism groups of simplicial complexes
as a matter of fact Sage doesn't call GAP here (nor anywhere else on graph automorphisms, in fact). GAP also doesn't do graph automorphsims "natively", it uses a package called GRAPE. Which calls nauty, in fact...
Sep
18
comment point in polytope
It's an honest approximation in the sense that you will get $\lambda$ with entries greater than certain positive $\epsilon$ (which you can compute from the size $S$ of the input) if $p$ is in the interior, else some entry of $\lambda$ will be smaller than $\epsilon$. And all this can be done in time polynomial in $S$.
Sep
18
comment point in polytope
If you don't use an interior point method then you have to use a simplex method, which in theory (and often in practice) is slower...
Sep
18
revised point in polytope
added 317 characters in body
Sep
18
comment point in polytope
you solve approximately, via an interior point method. There are bounds known. Actually, even better, if you're to use an interior method anyway, is to minimize some generic linear function on $\lambda$. The trick is that these methods converge to the so-called analytic center of the optimal face, and it's not so hard to see that the optimal $\lambda$ will be positive iff $p$ is interior.
Sep
17
answered point in polytope
Sep
17
comment point in polytope
Do you know if your $v_i$'s are in the convex position? Well, you can of course check this and get rid of these ones which are not first...
Sep
16
comment Presentation of the Monster Group
without the spider, you get some weird infinite Coxeter group.
Sep
15
comment Presentation of the Monster Group
dl.dropboxusercontent.com/u/11004520/CBO9780511629259A013.pdf
Sep
15
comment Presentation of the Monster Group
sure, but where to? your profile doesn't reveal any email...
Sep
15
comment Presentation of the Monster Group
@Derek: surely, one of main achievements of the theory of diagram geometries is construction of explicit presentations for most sporadic simple groups (perhaps their automorphism groups, sometimes). E.g. such a presentation is known for $M$.
Sep
15
answered Presentation of the Monster Group
Sep
12
comment Matrix power problem
indeed. I fixed the typo, thanks.
Sep
12
revised Matrix power problem
typo fixed
Sep
11
answered Matrix power problem
Sep
11
comment What is the independence number of hamming graph?
What is $H_q(n,d)$ ? Classically, $H(n,d)$ is the graph on $n$-tuples of words in the alphabet of size $d$, with adjacency being having Hamming distance 1. And why $q=2$? Do you mean $q=1$?