Category Archives: problem

2010-10 Metric space of matrices

Let  Mn×n be the space of real n×n matrices, regarded as a metric space with the distance function

\(\displaystyle d(A,B)=\sum_{i,j} |a_{ij}-b_{ij}|\)

for A=(aij) and B=(bij).
Prove that \(\{A\in M_{n\times n}: A^m=0 \text{ for some positive integer }m\}\) is a closed set.

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2010-9 No zeros far away

Let M>0 be a real number. Prove that there exists N so that if n>N, then all the roots of \(f_n(z)=1+\frac{1}{z}+\frac1{{2!}z^2}+\cdots+\frac{1}{n!z^n}\) are in the disk |z|<M on the complex plane.

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2010-8 Monochromatic Box

Let k be a postivive integer. Let f(k) be the minimum number n such that no matter how we color the integer points in {(x,y,z): 0<x,y,z≤n} with k colors, there always exist 8 monochromatic points forming the vertices of a box whose sides are parallel to xy- or yz- or xz- plane. Determine f(k).

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2010-7 Cardinality

Let S be the set of non-zero real numbers x such that there is exactly one 0-1 sequence {an} satisfying \(\displaystyle \sum_{n=1}^\infty a_n x^{-n}=1\). Prove that there is a one-to-one function from the set of all real numbers to S.

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