Question Number 676 by 123456 last updated on 22/Feb/15
![given two sequence a_n >0,b_n >0 such ∀n∈N^∗ ,a_n ^n <b_n <a_n ^(1/n) a. if Σ_(n=1) ^(+∞) a_n converge then Σ_(n=1) ^(+∞) b_n converge? b. proof that if a_n ∈(0,1) then b_n ∈(0,1) c. (dis)proof that if a_n →1 then b_n →1](https://www.tinkutara.com/question/Q676.png)
$${given}\:{two}\:{sequence}\:{a}_{{n}} >\mathrm{0},{b}_{{n}} >\mathrm{0}\:\:{such} \\ $$$$\forall{n}\in\mathbb{N}^{\ast} ,{a}_{{n}} ^{{n}} <{b}_{{n}} <{a}_{{n}} ^{\mathrm{1}/{n}} \\ $$$${a}.\:{if}\:\underset{{n}=\mathrm{1}} {\overset{+\infty} {\sum}}{a}_{{n}} \:{converge}\:{then}\:\underset{{n}=\mathrm{1}} {\overset{+\infty} {\sum}}{b}_{{n}} \:{converge}? \\ $$$${b}.\:{proof}\:{that}\:{if}\:{a}_{{n}} \in\left(\mathrm{0},\mathrm{1}\right)\:{then}\:{b}_{{n}} \in\left(\mathrm{0},\mathrm{1}\right) \\ $$$${c}.\:\left({dis}\right){proof}\:{that}\:{if}\:{a}_{{n}} \rightarrow\mathrm{1}\:{then}\:{b}_{{n}} \rightarrow\mathrm{1} \\ $$
Commented by prakash jain last updated on 22/Feb/15
![b. 0<a_n <1⇒0<a_n ^n <1 0<a_n <1⇒0<a_n ^(1/n) <1 a_n ^n <b_n <a_n ^(1/n) ⇒0<b_n <1 c. ∵ a_n →1⇒ a_n ^n →1 and a_n ^(1/n) →1 ∴ a_n →1⇒b_n →1 a. If a_n converges we can say a_n ^n converges. but we cannot conclude anything about a_n ^(1/n) . Hence we cannot conclude that b_n onverges.](https://www.tinkutara.com/question/Q678.png)
$$\mathrm{b}.\:\mathrm{0}<{a}_{{n}} <\mathrm{1}\Rightarrow\mathrm{0}<{a}_{{n}} ^{{n}} <\mathrm{1} \\ $$$$\:\:\:\:\:\mathrm{0}<{a}_{{n}} <\mathrm{1}\Rightarrow\mathrm{0}<{a}_{{n}} ^{\mathrm{1}/{n}} <\mathrm{1} \\ $$$$\:\:\:\:\:{a}_{{n}} ^{{n}} <{b}_{{n}} <{a}_{{n}} ^{\mathrm{1}/{n}} \Rightarrow\mathrm{0}<{b}_{{n}} <\mathrm{1} \\ $$$${c}.\:\because\:{a}_{{n}} \rightarrow\mathrm{1}\Rightarrow\:{a}_{{n}} ^{{n}} \rightarrow\mathrm{1}\:\mathrm{and}\:{a}_{{n}} ^{\mathrm{1}/{n}} \rightarrow\mathrm{1} \\ $$$$\:\:\:\:\:\:\therefore\:{a}_{{n}} \rightarrow\mathrm{1}\Rightarrow{b}_{{n}} \rightarrow\mathrm{1} \\ $$$${a}.\:\mathrm{If}\:{a}_{{n}} \:\mathrm{converges}\:\mathrm{we}\:\mathrm{can}\:\mathrm{say}\:{a}_{{n}} ^{{n}} \:\mathrm{converges}. \\ $$$$\:\:\:\:\:\:\mathrm{but}\:\mathrm{we}\:\mathrm{cannot}\:\mathrm{conclude}\:\mathrm{anything}\:\mathrm{about} \\ $$$$\:\:\:\:\:\:{a}_{{n}} ^{\mathrm{1}/{n}} .\:\mathrm{Hence}\:\mathrm{we}\:\mathrm{cannot}\:\mathrm{conclude}\:\mathrm{that}\:{b}_{{n}} \\ $$$$\:\:\:\:\:\:\mathrm{onverges}.\: \\ $$