Sunday, 27 January 2013

real analysis - Sequence defined by maxleftan,bnright. Proving convergence



Assume (an) and (bn) are two real sequences, and define cn=max{an,bn} for nN. Suppose (an) and (bn) are two convergent sequences. Prove then that (cn) is also a convergent sequence, and that lim



Attempt: (a_n) and (b_n) are both convergent. Hence there exists an n_0 \in \mathbb{N} such that \forall n \geq n_0: | a_n - L | < \epsilon. Furthermore, there exists an n_1 \in \mathbb{N} such that \forall n \geq n_1: | b_n - K | < \epsilon . Here L and K are the limits of resp. (a_n) and (b_n). Now let n_2 = \text{max} \left\{n_0, n_1 \right\}. Let n \geq n_2 be arbitrary. Then we have that L - \epsilon < a_n < L + \epsilon and K - \epsilon < b_n < K + \epsilon. Since c_n = \text{max}\left\{a_n, b_n\right\}, we have that c_n \geq a_n and c_n \geq b_n. So L - \epsilon < a_n \leq c_n and thus L - \epsilon < c_n.




But now I'm stuck. Help would be appreciated!


Answer



Remember that \max\{x,y\}=\frac{x+y+|x-y|}{2}.



Thus c_n=\frac{a_n+b_n+|a_n-b_n|}{2}. So, if \lim a_n=a and \lim b_n=b:



\lim c_n=\frac{1}{2}(\lim a_n+\lim b_n+|\lim a_n-\lim b_n|)=\frac{1}{2}(a+b+|a-b|)=\max\{a,b\}


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