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OLM Dolj Subiectul 1

Posted: Sat Feb 13, 2010 6:30 pm
by BogdanCNFB
Fie \( a,b\in \mathbb{R} \) si \( (a_n)_{n\ge 1},(b_n)_{n\ge 1} \) siruri de numere reale convergente la \( a \), respectiv \( b \).
Fie \( \sigma \) o permutare a numerelor naturale {\( 1,2,...,n \)}. Sa se arate ca \( \lim_{n\to\infty}\frac{\sum_{k=1}^n a_k b_{\sigma(k)}}{n}=ab \).

Posted: Tue Feb 16, 2010 12:47 am
by Marius Mainea
\( |\frac{a_1b_{\sigma(1)}+...+a_nb_{\sigma(n)}}{n}-ab|=|\frac{(a_1-a)b_{\sigma(1)}+...+(a_n-a)b_{\sigma(n)}}{n}+a\frac{(b_1-b)+(b_2-b)+...+(b_n-b)}{n}|\le M|\frac{(a_1-a)+...+(a_n-a)}{n}|+|a||\frac{(b_1-b)+(b_2-b)+...+(b_n-b)}{n}|\longrightarrow 0(n\to\infty) \)