Interesting Limit (28)

Calculus Level 4

lim n ln ( 1 2020 + 2 2020 + 3 2020 + + n 2020 ) ln n = ? \lim_{n\to\infty}\frac{\ln(1^{2020}+2^{2020}+3^{2020}+\cdots+n^{2020})}{\ln n}=\, ?


The answer is 2021.

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1 solution

Daniele Rosmondi
Dec 31, 2020

Here I used the fact that the result of k = 1 n k m \displaystyle \sum_{k=1}^n k^m can be expressed as a polynomial in n n of degree m + 1 m+1 and leading coefficient 1 m + 1 \frac 1{m+1} . Thus, it suffices to compute

lim n ln ( 1 2021 n 2021 ) ln n = \displaystyle \lim_{n\to\infty} \frac{\ln\Big(\frac 1{2021}n^{2021}\Big) } {\ln n} =

= lim n 2021 ln n ln 2021 ln n = \displaystyle =\lim_{n\to\infty} \frac{2021\ln n - \ln 2021} {\ln n} =

= lim n ( 2021 ln 2021 ln n ) = 2021 \displaystyle =\lim_{n\to\infty} \bigg(2021-\frac{\ln 2021} {\ln n}\bigg)=2021

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