Series Made by Limits

Calculus Level 4

k = 1 lim n 1 k + 2 k + + n k n k + 1 \large\sum_{k=1}^\infty\lim_{n\to\infty}\frac{1^k+2^k+\cdots+n^k}{n^{k+1}} Does the series converge?


Inspiration

Yes No

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1 solution

Brian Lie
Apr 13, 2018

Note that

L = lim n 1 k + 2 k + + n k n k + 1 = lim n 1 n i = 1 n ( i n ) k Using Riemann sums = 0 1 x k d x = x k + 1 k + 1 0 1 = 1 k + 1 , \begin{aligned} L&=\lim_{n\to\infty}\frac{1^k+2^k+\cdots+n^k}{n^{k+1}} \\&=\lim_{n\to\infty}\frac 1n\sum_{i=1}^n\left(\frac in\right)^k&\small\color{#3D99F6}\text{Using Riemann sums} \\&=\int_0^1x^k\ dx \\&=\frac {x^{k+1}}{k+1}\Bigg|_0^1 \\&=\frac 1{k+1}, \end{aligned}

we see that the series is actually harmonic series k = 1 1 k + 1 , \sum_{k=1}^\infty\frac 1{k+1}, which is divergent, making the answer No \boxed{\text{No}} .

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