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I am reading Houshang H. Sohrab's book: "Basic Real Analysis" (Second Edition).
I am focused on Chapter 2: Sequences and Series of Real Numbers ... ...
I need help with the proof of Theorem 2.3.9 (a) ...
Theorem 2.3.9 reads as follows:
View attachment 9065
Now, we can prove Theorem 2.3.9 (a) using the Cauchy Criterion for Series ... as follows:Since $$0 \leq x_n \leq y_n \ \forall \ n \in \mathbb{N}$$ ... we have that ...$$\left\vert \sum_{ k = n + 1 }^m x_n \right\vert \leq \left\vert \sum_{ k = n + 1 }^m y_n \right\vert$$ ... ... ... ... ... (1)But ... since $$ \sum_{ n = 1 }^{ \infty } y_n $$ is convergent, it satisfies the Cauchy Criterion for Series ...... further ... (1) implies that $$ \sum_{ n = 1 }^{ \infty } x_n $$ also satisfies the Cauchy Criterion for Series ... ... and so $$ \sum_{ n = 1 }^{ \infty } x_n $$ is convergent ...
However ... Sohrab states that Theorem 2.3.9 (a) is an immediate consequence of Theorem 2.3.6 (see below) ... ... this means we need to establish bounds for $$ \sum_{ n = 1 }^{ \infty } x_n $$ ...... can someone please demonstrate how to do this ...
Note: If $$\lim_{n \to \infty } t_n = \lim_{n \to \infty } \sum_{ k = 1 }^n y_k = L$$ ...... does $$L$$ form an upper bound on $$ \sum_{ n = 1 }^{ \infty } y_n $$ ...... and hence also form an upper bound on $$\sum_{ n = 1 }^{ \infty } x_n$$ ...... but ... how do we prove this ... ?
Help will be appreciated ...
Peter
=======================================================================================The post above refers to Theorem 2.3.6 ... so I am providing text of the same ... as follows:View attachment 9066
Hope that helps ...
Peter
I am focused on Chapter 2: Sequences and Series of Real Numbers ... ...
I need help with the proof of Theorem 2.3.9 (a) ...
Theorem 2.3.9 reads as follows:
View attachment 9065
Now, we can prove Theorem 2.3.9 (a) using the Cauchy Criterion for Series ... as follows:Since $$0 \leq x_n \leq y_n \ \forall \ n \in \mathbb{N}$$ ... we have that ...$$\left\vert \sum_{ k = n + 1 }^m x_n \right\vert \leq \left\vert \sum_{ k = n + 1 }^m y_n \right\vert$$ ... ... ... ... ... (1)But ... since $$ \sum_{ n = 1 }^{ \infty } y_n $$ is convergent, it satisfies the Cauchy Criterion for Series ...... further ... (1) implies that $$ \sum_{ n = 1 }^{ \infty } x_n $$ also satisfies the Cauchy Criterion for Series ... ... and so $$ \sum_{ n = 1 }^{ \infty } x_n $$ is convergent ...
However ... Sohrab states that Theorem 2.3.9 (a) is an immediate consequence of Theorem 2.3.6 (see below) ... ... this means we need to establish bounds for $$ \sum_{ n = 1 }^{ \infty } x_n $$ ...... can someone please demonstrate how to do this ...
Note: If $$\lim_{n \to \infty } t_n = \lim_{n \to \infty } \sum_{ k = 1 }^n y_k = L$$ ...... does $$L$$ form an upper bound on $$ \sum_{ n = 1 }^{ \infty } y_n $$ ...... and hence also form an upper bound on $$\sum_{ n = 1 }^{ \infty } x_n$$ ...... but ... how do we prove this ... ?
Help will be appreciated ...
Peter
=======================================================================================The post above refers to Theorem 2.3.6 ... so I am providing text of the same ... as follows:View attachment 9066
Hope that helps ...
Peter
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