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 Yearling
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Jan
22
comment Exact determinant of a circulant matrix
This determinant is also equal to the resultant of $X^n-1$ and $P:=c0+...+c_{n-1}X^{n-1}$. This resultant can be computed efficiently by modular algorithms (as mentionned on wikipedia for example, en.wikipedia.org/wiki/Resultants#Computation). There are also so-called subresultant algorithms (variation on using Euclid algorithm to compute the resultant but avoiding to work in the rationals).
Jan
5
comment Zeros of polynomials modulo non-prime
@user83633 : yes of course! I was not in a good day...
Dec
18
comment Zeros of polynomials modulo non-prime
My previous comment is wrong: over Z/n, one root of a polynomial gives a linear factor, but two distinct roots don't give a factorization by a product of two linear factors (due to the non integrity of the ring).
Dec
18
comment Zeros of polynomials modulo non-prime
Since roots give factorisation of polynomials even over the ring $\mathbf Z/n$, it is equivalent to search a characterisation of subsets $S \subset \mathbf Z/n$ s. t. for every $x$ not in $S$, $\prod_{s \in S} (x-s) \neq 0$.
Dec
18
comment Zeros of polynomials modulo non-prime
@user83633: as the OP suggests with her example mod 6, this is not as simple (due to the condition that $p$ must be non zero outside of $S$).
Dec
8
revised Concrete solution to the (oriented) Oberwolfach problem with one table
added 364 characters in body
Dec
8
comment Concrete solution to the (oriented) Oberwolfach problem with one table
Actually, there is a concrete solution for $n=8$ on page 3 of Bermond, J.-C.; Faber, V., Decomposition of the complete directed graph into k-circuits.J. Combinatorial Theory Ser. B 21 (1976), no. 2, 146–155. But they say it was found on a computer so it does not help that much.
Dec
8
comment Concrete solution to the (oriented) Oberwolfach problem with one table
One reference is : Alspach, Brian; Gavlas, Heather; Šajna, Mateja; Verrall, Helen Cycle decompositions. IV. Complete directed graphs and fixed length directed cycles. J. Combin. Theory Ser. A 103 (2003), no. 1, 165–208. But then one has to remount to older and older papers, treating special cases or equivalent problems.
Dec
7
comment Concrete solution to the (oriented) Oberwolfach problem with one table
There is of course a non-oriented variant of the problem. I'll be glad to hear about it as well.
Dec
7
revised Concrete solution to the (oriented) Oberwolfach problem with one table
added 119 characters in body
Dec
7
asked Concrete solution to the (oriented) Oberwolfach problem with one table
Nov
19
revised When is a submanifold of $\mathbf R^n$ given by global equations?
edited tags
Nov
18
revised Gysin exact sequence for a singular subvariety
added 7 characters in body
Nov
18
revised Gysin exact sequence for a singular subvariety
added 40 characters in body
Nov
18
revised Gysin exact sequence for a singular subvariety
edited tags
Nov
17
awarded  Yearling
Nov
17
asked Gysin exact sequence for a singular subvariety
Oct
9
awarded  Informed
Oct
9
comment Algebraic proof without using comparison theorem for étale cohomology
This fix looks very nice. I accepted the previous answer because it appeared first. Thank you anyway!
Oct
9
accepted Algebraic proof without using comparison theorem for étale cohomology