Where does it go?

Calculus Level 2

Will the sum of the sequence of 1 1 2 + 1 3 + 1 4 + 1+\dfrac{1}{2}+\dfrac{1}{3}+\dfrac{1}{4}+\cdots converge or diverge?

Converge Cannot be determined Diverge

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2 solutions

Margaret Zheng
Feb 11, 2016

Though the numerical value of this sequence tends to grow slowly, it actually diverges to infinity.

To prove this, first consider the sequence S1= 1 + 1 2 + 1 4 + 1 4 + 1 8 + 1 8 + 1 8 + 1 8 . . . 1+\frac{1}{2}+\frac{1}{4}+\frac{1}{4}+\frac{1}{8}+\frac{1}{8}+\frac{1}{8}+\frac{1}{8}... . Now rewrite it as S1 = 1 + 1 2 + 1 2 + 1 2 . . . 1+\frac{1}{2}+\frac{1}{2}+\frac{1}{2}... and apparently this sequence will diverge.

The last step is to compare S1 to our original sequence: Original sequence = 1 + 1 2 + 1 3 + 1 4 + 1 5 + 1 6 + 1 7 + 1 8 . . . 1+\frac{1}{2}+\frac{1}{3}+\frac{1}{4}+\frac{1}{5}+\frac{1}{6}+\frac{1}{7}+\frac{1}{8}... S1= 1 + 1 2 + 1 4 + 1 4 + 1 8 + 1 8 + 1 8 + 1 8 . . . 1+\frac{1}{2}+\frac{1}{4}+\frac{1}{4}+\frac{1}{8}+\frac{1}{8}+\frac{1}{8}+\frac{1}{8}...

Obviously, the original sequence appears to have larger value than S1. Since S1 diverges, this sequence must diverge too.

Moderator note:

Simple standard approach.

Kay Xspre
Feb 15, 2016

The Harmonic series, as this series are called, diverges . While Margaret provided simple proof, I will use the integral. For 1 1 2 + 1 3 + 1 4 + 1+\dfrac{1}{2}+\dfrac{1}{3}+\dfrac{1}{4}+\cdots This is equal to the area of rectangle of the size 1 × 1 n 1\times\frac{1}{n} . The area below the graph y = 1 x y = \frac{1}{x} from x = 1 x = 1 to n n is defined by 1 n d x x = l n ( n ) < 1 1 2 + 1 3 + 1 4 + + 1 n \int_{1}^{n}\frac{dx}{x} = ln(n) < 1+\dfrac{1}{2}+\dfrac{1}{3}+\dfrac{1}{4}+\cdots+\dfrac{1}{n} Here, as n n reaches infinity, the value of l n ( n ) ln(n) will reach its infinity as well, which, in turn, cause the series to diverge.

This is great and thank you very much!

Margaret Zheng - 5 years, 4 months ago

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