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Adelic integral factorization
This is treated in A. Weil's book on 'Adeles and Algebraic Groups'. In particular, see section(s up to) 3.4. 
Aug 20 
awarded  Yearling 
Aug 7 
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When is $(q^k1)/(q1)$ a perfect square?
I seemed to recall it being proved in Ribenboim's book about Catalan's conjecture (published before Mihailescu solved it), and, indeed, Shorey writes that it is (on p. 111, apparently) in his MathSciNet review of the book: ams.org/mathscinetgetitem?mr=95a:11029. 
Jun 30 
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Linear transformation of a polyhedron
Indeed, 'dualization' (polar reciprocation) provides a geometric proof of the equivalence, albeit a somewhat involved one. Off the top of my head, I think Stoer and Witzgall present that approach in their book on "Convexity and optimization" but I do not have it handy to check at the moment. 
Jun 29 
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Linear transformation of a polyhedron
What's wrong with the following? (Probably very close to what @Misha had in mind.) View a (not necessarily bounded) polytope as $\text{Conv}(\{u_i\}_{1 \le i \le k})+\text{Cone}(\{v_i\}_{1 \le i \le m})$ where $\text{Cone}(\{v_i\}_{1 \le i \le m}) = \left\{ \sum_{i=1}^m t_i v_i, \, t_i \ge 0 \right\}$. Now, if $L \in \text{End}(\mathbb{R}^n)$, then $L(P) = \text{Conv}(\{L(u_i)\}_{1 \le i \le k})+\text{Cone}(\{L(v_i)\}_{1 \le i \le m})$, which yields (1) (and so (2)). 
Oct 13 
awarded  Caucus 
Sep 6 
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Generalized quasiperfect numbers
I wondered about this a while ago and dblues had some nice preliminary observations in the very last post of this AoPS thread: artofproblemsolving.com/Forum/viewtopic.php?f=57&t=83696 
Jul 17 
awarded  Commentator 
Jul 17 
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Integerdistance sets
@Joseph O'Rourke: The answer is the same in $\mathbb{R}^d$ with $d \ge 3$. See e.g. the original paper by N. H. Anning and P. ErdÅ‘s: ams.org/journals/bull/19455108/S000299041945084079/… 
Jul 17 
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Integerdistance sets
@Carl: It is not an unsolved problem anymore. Such a set has been constructed by T. Kreisel and S. Kurz in 2008: arxiv.org/abs/0804.1303v1 
Jul 16 
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Generalization of Rado's Single Equation theorem
See also this nicely presented expository note by T. Tao (p. 12 in particular): math.ucla.edu/~tao/preprints/Expository/ramsey.dvi 
Jun 6 
awarded  Nice Answer 
Apr 2 
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sum of digits in different bases
For 1000: our maximal $n$ must satisfy $(s_10(n)=) s_2(n) \le 9$. There are only 5 numbers with 9 ones in binary  none of them works. Then check those numbers whose decimal sum is 8 (800, 710, etc.). None of them work until 503, which is the answer in this case. Granted, this is still somewhat brute force... 
Mar 22 
answered  Rational points on a sphere in $\mathbb{R}^d$ 
Jun 16 
awarded  Enthusiast 
Jun 7 
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determinants and polynomials in matrices
For discussions of it, I highly recommend the following three threads on the AoPS forum: artofproblemsolving.com/Forum/viewtopic.php?f=349&t=43292 artofproblemsolving.com/Forum/viewtopic.php?f=349&t=218429 artofproblemsolving.com/Forum/viewtopic.php?f=349&t=279631 
Jun 4 
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Formula in common: How to search for same/similar equations in other knowledge domains?
There was a long discussion touching on that issue over at Tim Gowers's blog: gowers.wordpress.com/2012/03/21/… 
May 29 
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Bounding a signed sum of complex numbers
There is no need to (re)post it on the AoPS forum: artofproblemsolving.com/Forum/viewtopic.php?f=42&t=386586 
May 20 
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Estimate for products of integers that are relatively prime with $N$
Yes, mod N indeed, thank you. 
May 20 
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Elementary applications of KreinMilman
A reference for that proof is the book "A Problem Seminar" by Donald J. Newman. 