2008
Problem - 3269
Let $$f(r) = \displaystyle\sum_{j=2}^{2008}\frac{1}{j^r} = \frac{1}{2^r}+\frac{1}{3^r}+\cdots+\frac{1}{2016^r}$$
Find $$\sum_{k=2}^{\infty}f(k)$$
First, expand $\displaystyle\sum_{k=2}^{\infty}f(k)$:
$$
\begin{array}{ccccccccccc}
f(2) &=& \frac{1}{2^2} &+& \frac{1}{3^2} &+& \frac{1}{4^2} &+& \cdots &+& \frac{1}{2016^2}\\
f(3) &=& \frac{1}{2^3} &+& \frac{1}{3^3} &+& \frac{1}{4^3} &+& \cdots &+& \frac{1}{2016^3}\\
f(4) &=& \frac{1}{2^4} &+& \frac{1}{3^4} &+& \frac{1}{4^4} &+& \cdots &+& \frac{1}{2016^4}\\
&\cdots
\end{array}
$$
Next, add these terms by column. Each forms an infinite geometric sequence.
$$
\begin{array}{ccccc}
\frac{1}{2^2} + \frac{1}{2^3} + \frac{1}{2^4} + \cdots &=& \frac{1}{2^2}\times\frac{1}{1-\frac{1}{2}}&=&\frac{1}{1\times 2}\\
\frac{1}{3^2} + \frac{1}{3^3} + \frac{1}{3^4} + \cdots &=& \frac{1}{3^2}\times\frac{1}{1-\frac{1}{3}}&=&\frac{1}{2\times 3}\\
\frac{1}{4^2} + \frac{1}{4^3} + \frac{1}{4^4} + \cdots &=& \frac{1}{4^2}\times\frac{1}{1-\frac{1}{4}}&=&\frac{1}{3\times 4}\\
\cdots\\
\frac{1}{2016^2} + \frac{1}{2016^3} + \frac{1}{2016^4} + \cdots &=& \frac{1}{2016^2}\times\frac{1}{1-\frac{1}{2016}}&=&\frac{1}{2015\times 2016}\\
\end{array}
$$
Therefore, the sum equals
\begin{align*}
&\frac{1}{1\times 2}+\frac{1}{2\times 3}+\frac{1}{3\times 4}+\cdots+\frac{1}{2015\times 2016}\\
=& \Big(\frac{1}{1}-\frac{1}{2}\Big)+\Big(\frac{1}{2}-\frac{1}{3}\Big)+\cdots +\Big(\frac{1}{2015}-\frac{1}{2016}\Big)\\
=& \boxed{\frac{2015}{2016}}
\end{align*}