# Is-the-sequence-u-n-defined-by-the-formula-u-n-n-1-5-n-lgn-where-n-Z-n-2-convergent-If-so-find-its-sum-as-n-

Question Number 408 by 112358 last updated on 30/Dec/14
$${Is}\:{the}\:{sequence}\:\left\{{u}_{{n}} \right\}\:{defined}\:{by}\:{the}\:{formula}\:{u}_{{n}} =\frac{{n}+\left(−\mathrm{1}.\mathrm{5}\right)^{{n}} }{{lgn}}\:,\:{where}\:{n}\in{Z}^{+} \mid{n}\geqslant\mathrm{2}\:,\:{convergent}?\:{If}\:{so}\:{find}\:{its}\:{sum}\:{as}\:{n}\rightarrow\infty. \\$$
Commented by 123456 last updated on 30/Dec/14
$${u}_{{n}} =\frac{{n}+\left(−\mathrm{1}.\mathrm{5}\right)^{{n}} }{\mathrm{lg}\:\mathrm{n}\:},{n}\in\mathbb{N},{n}\geqslant\mathrm{2} \\$$
Commented by 123456 last updated on 30/Dec/14
$$\frac{\infty\pm\infty}{\infty} \\$$$$\frac{?}{\infty},\frac{\infty}{\infty} \\$$
Commented by prakash jain last updated on 31/Dec/14
$${u}_{{n}} =\frac{{n}+\left(−\mathrm{1}.\mathrm{5}\right)^{{n}} }{\mathrm{lg}\:{n}} \\$$$${u}_{\mathrm{2}{k}} =\frac{\mathrm{2}{k}+\left(\mathrm{1}.\mathrm{5}\right)^{\mathrm{2}{k}} }{\mathrm{lg}\:\mathrm{2}{k}} \\$$$${u}_{\mathrm{2}{k}+\mathrm{1}} =\frac{\left(\mathrm{2}{k}+\mathrm{1}\right)−\left(\mathrm{1}.\mathrm{5}\right)^{\mathrm{2}{k}+\mathrm{1}} }{\mathrm{lg}\:\left(\mathrm{2}{k}+\mathrm{1}\right)} \\$$$$\mathrm{We}\:\mathrm{can}\:\mathrm{define}\:\mathrm{a}\:\mathrm{sequence}\:\mathrm{with}\:{i}^{{th}\:} \mathrm{element}\:{v}_{{i}} \\$$$$\mathrm{given}\:\mathrm{by}\:{i}\geqslant\mathrm{1} \\$$$${v}_{{i}} ={u}_{\mathrm{2}{i}} +{u}_{\mathrm{2}{i}+\mathrm{1}} \\$$$${v}_{{i}} =\frac{\mathrm{2}{i}}{\mathrm{lg}\:\mathrm{2}{i}}+\frac{\mathrm{2}{i}+\mathrm{1}}{\mathrm{lg}\:\left(\mathrm{2}{i}+\mathrm{1}\right)}+\frac{\left(\mathrm{1}.\mathrm{5}\right)^{\mathrm{2}{i}} }{\mathrm{lg}\:\mathrm{2}{i}}−\frac{\left(\mathrm{1}.\mathrm{5}\right)^{\mathrm{2}{i}+\mathrm{1}} }{\mathrm{lg}\:\left(\mathrm{2}{i}+\mathrm{1}\right)} \\$$