Those fractions each tend to 0

Calculus Level 3

lim n i = 1 n 1 n + i \large \displaystyle \lim_{n \to \infty} \displaystyle \sum_{i=1}^n \dfrac {1}{n+i}

Find the value of the closed form of the above limit to 2 decimal places.


The answer is 0.69.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Rohith M.Athreya
Jan 21, 2017

Relevant wiki: Riemann Sums

alternatively,

lim n i = 1 n 1 n + i = 1 n i = 1 n 1 1 + i n = 1 2 d t t = ln 2 \large \displaystyle \lim_{n \to \infty} \displaystyle \sum_{i=1}^n \dfrac {1}{n+i}= \frac{1}{n} \sum_{i=1}^{n} \frac{1}{1+\frac{i}{n}}=\int_{1}^{2}\frac{dt}{t}=\ln2

Sharky Kesa
Jan 21, 2017

The summation can be rewritten as H ( 2 n ) H ( n ) H(2n) - H(n) , where H ( n ) = i = 1 n 1 i H(n)=\displaystyle \sum_{i=1}^{n} \frac{1}{i} . Thus, we wish to find

L = lim n H ( 2 n ) H ( n ) L = \displaystyle \lim_{n \to \infty} H(2n) - H(n)

Note that the Euler-Mascheroni constant is defined as

γ = lim n H ( n ) ln ( n ) \gamma = \displaystyle \lim_{n \to \infty} H(n)-\ln(n)

Thus,

L = lim n ln ( 2 n ) ln ( n ) = ln ( 2 ) = 0.69 L = \displaystyle \lim_{n \to \infty} \ln(2n)-\ln(n) = \ln(2) = 0.69

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...