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Apr 5 at 6:35 history edited Pietro Majer CC BY-SA 4.0
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Nov 15, 2018 at 20:23 comment added Dima Pasechnik Harmonic $n$-variate polynomials of a given degree are $S_n$-invariant (as a set), and thus one could come up with an $S_n$-invariant basis, but the corresponding action won't be so simple.
Apr 20, 2018 at 7:42 vote accept Pietro Majer
Apr 13, 2017 at 12:57 history edited CommunityBot
replaced http://mathoverflow.net/ with https://mathoverflow.net/
Sep 4, 2013 at 20:03 history edited Yemon Choi CC BY-SA 3.0
as we are fixing broken rendering on very old posts, I've fixed a typo in the title and cleaned up some English
Sep 4, 2013 at 13:59 history edited Pietro Majer CC BY-SA 3.0
deleted 1 characters in body
Oct 20, 2011 at 22:42 answer added Qiaochu Yuan timeline score: 7
Oct 20, 2011 at 22:25 comment added Qiaochu Yuan Sorry if I am being silly, but I don't see how the result you cite can be true for $n = 2$.
Oct 20, 2011 at 19:55 comment added Pietro Majer Thank you... So, the existence of an invariant basis may really depend on the pair $(n,m)$ ?
Oct 20, 2011 at 19:45 comment added Qiaochu Yuan If the result is false it can be disproved with a finite, albeit tedious, calculation. A finite-dimensional real vector space on which $S_n$ acts admits a permutation-invariant basis if and only if it is a direct sum of permutation representations, and for fixed $n$ this can be checked by comparing the character of the representation against the characters of all permutation representations (this is the tedious part). In particular the number of copies of the trivial representation is the number of orbits of the action of $S_n$, so if there are no such copies then the result is false.
Oct 20, 2011 at 19:12 history asked Pietro Majer CC BY-SA 3.0