In mathematics, the harmonic series is the divergent infinite series:
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.
The fact that the harmonic series diverges was first proven in the 14th century by Nicole Oresme,^{[1]} but this achievement fell into obscurity. Proofs were given in the 17th century by Pietro Mengoli,^{[2]} Johann Bernoulli,^{[3]} and Jacob Bernoulli.^{[4]}^{[5]}
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.^{[6]}
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".^{[7]} Suppose that a worm crawls along an infinitelyelastic onemeter 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
(In fact the actual ratio is a little less than this sum as the band expands continuously.) The reason is that the band also expands behind the worm; 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 10^{43} minutes (10^{37} years). Although the harmonic series does diverge, it does so very slowly.
Another problem involving the harmonic series is the Jeep problem.
Another example is the blockstacking 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.^{[7]}^{[8]}
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 10meter 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):
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.
There are several wellknown proofs of the divergence of the harmonic series. A few of them are given below.
One way to prove divergence is to compare the harmonic series with another divergent series, where each denominator is replaced with the nextlargest power of two:
Each term of the harmonic series is greater than or equal to the corresponding term of the second series, and therefore the sum of the harmonic series must be greater than the sum of the second series. However, the sum of the second series is infinite:
It follows (by the comparison test) that the sum of the harmonic series must be infinite as well. More precisely, the comparison above proves that
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.
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:
Additionally, the total area under the curve y = 1/x from 1 to infinity is given by a divergent improper integral:
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
The generalization of this argument is known as the integral test.
The harmonic series diverges very slowly. For example, the sum of the first 10^{43} terms is less than 100.^{[9]} This is because the partial sums of the series have logarithmic growth. In particular,
where γ is the Euler–Mascheroni constant and ε_{k} ~ 1/2k which approaches 0 as k goes to infinity. 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.
The first thirty harmonic numbers  

n  Partial sum of the harmonic series, H_{n}  
expressed as a fraction  decimal  relative size  
1  1  1 


2  3  /2  1.5 

3  11  /6  ~1.83333 

4  25  /12  ~2.08333 

5  137  /60  ~2.28333 

6  49  /20  2.45 

7  363  /140  ~2.59286 

8  761  /280  ~2.71786 

9  7129  /2520  ~2.82897 

10  7381  /2520  ~2.92897 

11  83711  /27720  ~3.01988 

12  86021  /27720  ~3.10321 

13  1145993  /360360  ~3.18013 

14  1171733  /360360  ~3.25156 

15  1195757  /360360  ~3.31823 

16  2436559  /720720  ~3.38073 

17  42142223  /12252240  ~3.43955 

18  14274301  /4084080  ~3.49511 

19  275295799  /77597520  ~3.54774 

20  55835135  /15519504  ~3.59774 

21  18858053  /5173168  ~3.64536 

22  19093197  /5173168  ~3.69081 

23  444316699  /118982864  ~3.73429 

24  1347822955  /356948592  ~3.77596 

25  34052522467  /8923714800  ~3.81596 

26  34395742267  /8923714800  ~3.85442 

27  312536252003  /80313433200  ~3.89146 

28  315404588903  /80313433200  ~3.92717 

29  9227046511387  /2329089562800  ~3.96165 

30  9304682830147  /2329089562800  ~3.99499 

The finite partial sums of the diverging harmonic series,
are called harmonic numbers.
The difference between H_{n} 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 H_{1} = 1.^{[10]}^{:p. 24}^{[11]}^{:Thm. 1}
The series
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:
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:
This is known as the Leibniz series.
The general harmonic series is of the form
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.
A generalization of the harmonic series is the pseries (or hyperharmonic series), defined as
for any positive real number p. When p = 1, the pseries is the harmonic series, which diverges. Either the integral test or the Cauchy condensation test shows that the pseries converges for all p > 1 (in which case it is called the overharmonic series) and diverges for all p ≤ 1. If p > 1 then the sum of the pseries is ζ(p), i.e., the Riemann zeta function evaluated at p.
The problem of finding the sum for p = 2 is called the Basel problem; Leonhard Euler showed it is π^{2}/6. The value of the sum for p = 3 is called Apéry's constant.
Related to the pseries is the lnseries, defined as
for any positive real number p. This can be shown by the integral test to diverge for p ≤ 1 but converge for all p > 1.
For any convex, realvalued function φ such that
the series is convergent.
The random harmonic series
where the s_{n} are independent, identically distributed random variables taking the values +1 and −1 with equal probability 1/2, is a wellknown example in probability theory for a series of random variables that converges with probability 1. The fact of this convergence is an easy consequence of either the Kolmogorov threeseries theorem or of the closely related Kolmogorov maximal inequality. Byron Schmuland of the University of Alberta further examined^{[12]} the properties of the random harmonic series, and showed that the convergent is a random variable with some interesting properties. In particular, the probability density function of this random variable evaluated at +2 or at −2 takes on the value 0.124999999999999999999999999999999999999999764…, differing from 1/8 by less than 10^{−42}. Schmuland's paper explains why this probability is so close to, but not exactly, 1/8. The exact value of this probability is given by the infinite cosine product integral C_{2}^{[13]} divided by π.
The depleted harmonic series where all of the terms in which the digit 9 appears anywhere in the denominator are removed can be shown to converge and its value is less than 80.^{[14]} In fact, when all the terms containing any particular string of digits (in any base) are removed the series converges.
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