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In mathematics , the HARMONIC SERIES is the divergent infinite series : n = 1 1 n = 1 + 1 2 + 1 3 + 1 4 + 1 5 + {displaystyle sum _{n=1}^{infty }{frac {1}{n}}=1+{frac {1}{2}}+{frac {1}{3}}+{frac {1}{4}}+{frac {1}{5}}+cdots }

Its name derives from the concept of overtones , or harmonics in music : the wavelengths of the overtones of a vibrating string are 1/2, 1/3, 1/4, etc., of the string's fundamental wavelength . Every term of the series after the first is the harmonic mean of the neighboring terms; the phrase harmonic mean likewise derives from music.

CONTENTS

* 1 History * 2 Paradoxes

* 3 Divergence
Divergence

* 3.1 Comparison test * 3.2 Integral
Integral
test

* 4 Rate of divergence * 5 Partial sums

* 6 Related series

* 6.1 Alternating harmonic series * 6.2 General harmonic series * 6.3 p-series * 6.4 ln-series * 6.5 φ-series * 6.6 Random harmonic series * 6.7 Depleted harmonic series

* 7 See also * 8 References * 9 External links

HISTORY

The fact that the harmonic series diverges was first proven in the 14th century by Nicole Oresme , but this achievement fell into obscurity. Proofs were given in the 17th century by Pietro Mengoli , Johann Bernoulli , and Jacob Bernoulli .

Historically, harmonic sequences have had a certain popularity with architects. This was so particularly in the Baroque period, when architects used them to establish the proportions of floor plans , of elevations , and to establish harmonic relationships between both interior and exterior architectural details of churches and palaces.

PARADOXES

The harmonic series can be counterintuitive to students first encountering it, because it is a divergent series even though the limit of the nth term as n goes to infinity is zero. The divergence of the harmonic series is also the source of some apparent paradoxes . One example of these is the "worm on the rubber band ". Suppose that a worm crawls along an infinitely-elastic one-meter rubber band at the same time as the rubber band is uniformly stretched. If the worm travels 1 centimeter per minute and the band stretches 1 meter per minute, will the worm ever reach the end of the rubber band? The answer, counterintuitively, is "yes", for after n minutes, the ratio of the distance travelled by the worm to the total length of the rubber band is 1 100 k = 1 n 1 k {displaystyle {frac {1}{100}}sum _{k=1}^{n}{frac {1}{k}}}

(In fact the actual ratio is a little less than this sum as the band expands continuously.) The reason is that the band expands behind the worm also; eventually, the worm gets past the midway mark and the band behind expands increasingly more rapidly than the band in front.

Because the series gets arbitrarily large as n becomes larger, eventually this ratio must exceed 1, which implies that the worm reaches the end of the rubber band. However, the value of n at which this occurs must be extremely large: approximately e 100, a number exceeding 1043 minutes (1037 years). Although the harmonic series does diverge, it does so very slowly.

Another problem involving the harmonic series is the Jeep problem . The block-stacking problem : blocks aligned according to the harmonic series bridges cleavages of any width.

Another example is the block-stacking problem : given a collection of identical dominoes, it is clearly possible to stack them at the edge of a table so that they hang over the edge of the table without falling. The counterintuitive result is that one can stack them in such a way as to make the overhang arbitrarily large, provided there are enough dominoes.

A simpler example, on the other hand, is the swimmer that keeps adding more speed when touching the walls of the pool. The swimmer starts crossing a 10-meter pool at a speed of 2 m/s, and with every cross, another 2 m/s is added to the speed. In theory, the swimmer's speed is unlimited, but the number of pool crosses needed to get to that speed becomes very large; for instance, to get to the speed of light (ignoring special relativity ), the swimmer needs to cross the pool 150 million times. Contrary to this large number, the time required to reach a given speed depends on the sum of the series at any given number of pool crosses (iterations): 10 2 k = 1 n 1 k . {displaystyle {frac {10}{2}}sum _{k=1}^{n}{frac {1}{k}}.}

Calculating the sum (iteratively) shows that to get to the speed of light the time required is only 94 seconds. By continuing beyond this point (exceeding the speed of light, again ignoring special relativity ), the time taken to cross the pool will in fact approach zero as the number of iterations becomes very large, and although the time required to cross the pool appears to tend to zero (at an infinite number of iterations), the sum of iterations (time taken for total pool crosses) will still diverge at a very slow rate.

DIVERGENCE

There are several well-known proofs of the divergence of the harmonic series. A few of them are given below.

COMPARISON TEST

One way to prove divergence is to compare the harmonic series with another divergent series, where each denominator is replaced with the next-largest power of two : 1 + 1 2 + 1 3 + 1 4 + 1 5 + 1 6 + 1 7 + 1 8 + 1 9 + > 1 + 1 2 + 1 4 + 1 4 + 1 8 + 1 8 + 1 8 + 1 8 + 1 16 + {displaystyle {begin{aligned}&{}1+{frac {1}{2}}+{frac {1}{3}}+{frac {1}{4}}+{frac {1}{5}}+{frac {1}{6}}+{frac {1}{7}}+{frac {1}{8}}+{frac {1}{9}}+cdots \>{} width:50.025ex; height:13.676ex;" alt="{displaystyle {begin{aligned}&{}1+{frac {1}{2}}+{frac {1}{3}}+{frac {1}{4}}+{frac {1}{5}}+{frac {1}{6}}+{frac {1}{7}}+{frac {1}{8}}+{frac {1}{9}}+cdots \>{}"> 1 + ( 1 2 ) + ( 1 4 + 1 4 ) + ( 1 8 + 1 8 + 1 8 + 1 8 ) + ( 1 16 + + 1 16 ) + = 1 + 1 2 + 1 2 + 1 2 + 1 2 + = {displaystyle {begin{aligned}&{}1+left({frac {1}{2}}right)+left({frac {1}{4}}+{frac {1}{4}}right)+left({frac {1}{8}}+{frac {1}{8}}+{frac {1}{8}}+{frac {1}{8}}right)+left({frac {1}{16}}+cdots +{frac {1}{16}}right)+cdots \={} width:75.41ex; height:14.509ex;" alt="{displaystyle {begin{aligned}&{}1+left({frac {1}{2}}right)+left({frac {1}{4}}+{frac {1}{4}}right)+left({frac {1}{8}}+{frac {1}{8}}+{frac {1}{8}}+{frac {1}{8}}right)+left({frac {1}{16}}+cdots +{frac {1}{16}}right)+cdots \={}"> n = 1 2 k 1 n 1 + k 2 {displaystyle sum _{n=1}^{2^{k}}{frac {1}{n}}geq 1+{frac {k}{2}}}

for every positive integer k.

This proof, proposed by Nicole Oresme in around 1350, is considered by many in the mathematical community to be a high point of medieval mathematics . It is still a standard proof taught in mathematics classes today. Cauchy\'s condensation test is a generalization of this argument.

INTEGRAL TEST

Illustration of the integral test.

It is possible to prove that the harmonic series diverges by comparing its sum with an improper integral . Specifically, consider the arrangement of rectangles shown in the figure to the right. Each rectangle is 1 unit wide and 1/n units high, so the total area of the infinite number of rectangles is the sum of the harmonic series: area of rectangles = 1 + 1 2 + 1 3 + 1 4 + 1 5 + {displaystyle {begin{array}{c}{text{area of}}\{text{rectangles}}end{array}}=1+{frac {1}{2}}+{frac {1}{3}}+{frac {1}{4}}+{frac {1}{5}}+cdots }

Additionally, the total area under the curve y = 1/x from 1 to infinity is given by a divergent improper integral : area under curve = 1 1 x d x = . {displaystyle {begin{array}{c}{text{area under}}\{text{curve}}end{array}}=int _{1}^{infty }{frac {1}{x}},dx=infty .}

Since this area is entirely contained within the rectangles, the total area of the rectangles must be infinite as well. More precisely, this proves that n = 1 k 1 n > 1 k + 1 1 x d x = ln ( k + 1 ) . {displaystyle sum _{n=1}^{k}{frac {1}{n}}>int _{1}^{k+1}{frac {1}{x}},dx=ln(k+1).}

The generalization of this argument is known as the integral test .

RATE OF DIVERGENCE

The harmonic series diverges very slowly. For example, the sum of the first 1043 terms is less than 100. This is because the partial sums of the series have logarithmic growth . In particular, n = 1 k 1 n = ln k + + k ( ln k ) + 1 {displaystyle sum _{n=1}^{k}{frac {1}{n}}=ln k+gamma +varepsilon _{k}leq (ln k)+1}

where γ is the Euler–Mascheroni constant and εk ~ 1/2k which approaches 0 as k goes to infinity. Leonhard Euler
Leonhard Euler
proved both this and also the more striking fact that the sum which includes only the reciprocals of primes also diverges, i.e. p prime 1 p = 1 2 + 1 3 + 1 5 + 1 7 + 1 11 + 1 13 + 1 17 + = . {displaystyle sum _{p{text{ prime }}}{frac {1}{p}}={frac {1}{2}}+{frac {1}{3}}+{frac {1}{5}}+{frac {1}{7}}+{frac {1}{11}}+{frac {1}{13}}+{frac {1}{17}}+cdots =infty .}

PARTIAL SUMS

The first thirty harmonic numbers N PARTIAL SUM OF THE HARMONIC SERIES, HN

EXPRESSED AS A FRACTION DECIMAL RELATIVE SIZE

1 1 ~1 1

2 3 /2 ~1.5 1.5

3 11 /6 ~1.83333 1.83333

4 25 /12 ~2.08333 2.08333

5 137 /60 ~2.28333 2.28333

6 49 /20 ~2.45 2.45

7 363 /140 ~2.59286 2.59286

8 761 /280 ~2.71786 2.71786

9 7003712900000000000♠7129 /7003252000000000000♠2520 ~2.82897 2.82897

10 7003738100000000000♠7381 /7003252000000000000♠2520 ~2.92897 2.92897

11 7004837110000000000♠83711 /7004277200000000000♠27720 ~3.01988 3.01988

12 7004860210000000000♠86021 /7004277200000000000♠27720 ~3.10321 3.10321

13 7006114599300000000♠1145993 /7005360360000000000♠360360 ~3.18013 3.18013

14 7006117173300000000♠1171733 /7005360360000000000♠360360 ~3.25156 3.25156

15 7006119575700000000♠1195757 /7005360360000000000♠360360 ~3.31823 3.31823

16 7006243655900000000♠2436559 /7005720720000000000♠720720 ~3.38073 3.38073

17 7007421422230000000♠42142223 /7007122522400000000♠12252240 ~3.43955 3.43955

18 7007142743010000000♠14274301 /7006408408000000000♠4084080 ~3.49511 3.49511

19 7008275295799000000♠275295799 /7007775975200000000♠77597520 ~3.54774 3.54774

20 7007558351350000000♠55835135 /7007155195040000000♠15519504 ~3.59774 3.59774

21 7007188580530000000♠18858053 /7006517316800000000♠5173168 ~3.64536 3.64536

22 7007190931970000000♠19093197 /7006517316800000000♠5173168 ~3.69081 3.69081

23 7008444316699000000♠444316699 /7008118982864000000♠118982864 ~3.73429 3.73429

24 7009134782295500000♠1347822955 /7008356948592000000♠356948592 ~3.77596 3.77596

25 7010340525224670000♠34052522467 /7009892371480000000♠8923714800 ~3.81596 3.81596

26 7010343957422670000♠34395742267 /7009892371480000000♠8923714800 ~3.85442 3.85442

27 7011312536252003000♠312536252003 /7010803134332000000♠80313433200 ~3.89146 3.89146

28 7011315404588903000♠315404588903 /7010803134332000000♠80313433200 ~3.92717 3.92717

29 7012922704651138700♠9227046511387 /7012232908956280000♠2329089562800 ~3.96165 3.96165

30 7012930468283014700♠9304682830147 /7012232908956280000♠2329089562800 ~3.99499 3.99499

The nth partial sum of the diverging harmonic series, H n = k = 1 n 1 k , {displaystyle H_{n}=sum _{k=1}^{n}{frac {1}{k}},}

is called the nth HARMONIC NUMBER .

The difference between Hn and ln n converges to the Euler–Mascheroni constant . The difference between any two harmonic numbers is never an integer. No harmonic numbers are integers, except for H1 = 1.

RELATED SERIES

ALTERNATING HARMONIC SERIES

See also: Riemann series theorem § Changing the sum The first fourteen partial sums of the alternating harmonic series (black line segments) shown converging to the natural logarithm of 2 (red line).

The series n = 1 ( 1 ) n + 1 n = 1 1 2 + 1 3 1 4 + 1 5 {displaystyle sum _{n=1}^{infty }{frac {(-1)^{n+1}}{n}}=1-{frac {1}{2}}+{frac {1}{3}}-{frac {1}{4}}+{frac {1}{5}}-cdots }

is known as the ALTERNATING HARMONIC SERIES. This series converges by the alternating series test . In particular, the sum is equal to the natural logarithm of 2 : 1 1 2 + 1 3 1 4 + 1 5 = ln 2. {displaystyle 1-{frac {1}{2}}+{frac {1}{3}}-{frac {1}{4}}+{frac {1}{5}}-cdots =ln 2.}

The alternating harmonic series, while conditionally convergent , is not absolutely convergent : if the terms in the series are systematically rearranged, in general the sum becomes different and, dependent on the rearrangement, possibly even infinite.

The alternating harmonic series formula is a special case of the Mercator series , the Taylor series for the natural logarithm.

A related series can be derived from the Taylor series for the arctangent : n = 0 ( 1 ) n 2 n + 1 = 1 1 3 + 1 5 1 7 + = 4 . {displaystyle sum _{n=0}^{infty }{frac {(-1)^{n}}{2n+1}}=1-{frac {1}{3}}+{frac {1}{5}}-{frac {1}{7}}+cdots ={frac {pi }{4}}.}

This is known as the Leibniz series .

GENERAL HARMONIC SERIES

The GENERAL HARMONIC SERIES is of the form n = 0 1 a n + b , {displaystyle sum _{n=0}^{infty }{frac {1}{an+b}},}

where a ≠ 0 and b are real numbers and b/a is not a nonpositive integer.

By the limit comparison test with the harmonic series, all general harmonic series also diverge.

P-SERIES

A generalization of the harmonic series is the P-SERIES (or HYPERHARMONIC SERIES), defined as n = 1 1 n p {displaystyle sum _{n=1}^{infty }{frac {1}{n^{p}}}}

for any positive real number p. When p = 1, the p-series is the harmonic series, which diverges. Either the integral test or the Cauchy condensation test shows that the p-series converges for all p