A harmonic number is a number of the form
(1)

arising from truncation of the harmonic series. A harmonic number can be expressed analytically as
(2)

where is the EulerMascheroni constant and is the digamma function.
The first few harmonic numbers are 1, , , , , ... (OEIS A001008 and A002805). The numbers of digits in the numerator of for , 1, ... are 1, 4, 41, 434, 4346, 43451, 434111, 4342303, 43428680, ... (OEIS A114467), with the corresponding number of digits in the denominator given by 1, 4, 40, 433, 4345, 43450, 434110, 4342302, 43428678, ... (OEIS A114468). These digits converge to what appears to be the decimal digits of (OEIS A002285).
The first few indices such that the numerator of is prime are given by 2, 3, 5, 8, 9, 21, 26, 41, 56, 62, 69, ... (OEIS A056903). The search for prime numerators has been completed up to by E. W. Weisstein (May 13, 2009), and the following table summarizes the largest known values.
decimal digits  discoverer  
63942  27795  E. W. Weisstein (Feb. 14, 2007) 
69294  30067  E. W. Weisstein (Feb. 1, 2008) 
69927  30301  E. W. Weisstein (Mar. 11, 2008) 
77449  33616  E. W. Weisstein (Apr. 4, 2009) 
78128  33928  E. W. Weisstein (Apr. 9, 2009) 
78993  34296  E. W. Weisstein (Apr. 17, 2009) 
81658  35479  E. W. Weisstein (May. 12, 2009) 
The denominators of appear never to be prime except for the case . Furthermore, the denominator is never a prime power (except for this case) since the denominator is always divisible by the largest power of 2 less than or equal to , and also by any prime with .
The harmonic numbers are implemented as HarmonicNumber[n].
The values of such that equals or exceeds 1, 2, 3, ... are given by 1, 4, 11, 31, 83, 227, 616, 1674, ... (OEIS A004080). Another interesting sequence is the number of terms in the simple continued fraction of for , 1, 2, ..., given by 1, 8, 68, 834, 8356, 84548, 841817, 8425934, 84277586, ... (OEIS A091590), which is conjectured to approach (OEIS A089729).
The definition of harmonic numbers can also be extended to the complex plane, as illustrated above.
Based on their definition, harmonic numbers satisfy the obvious recurrence equation
(3)

with .
The number formed by taking alternate signs in the sum also has an explicit analytic form
(4)
 
(5)
 
(6)

has the particularly beautiful form
(7)
 
(8)
 
(9)
 
(10)
 
(11)
 
(12)

The harmonic number is never an integer except for , which can be proved by using the strong triangle inequality to show that the 2adic value of is greater than 1 for . This result was proved in 1915 by Taeisinger, and the more general results that any number of consecutive terms not necessarily starting with 1 never sum to an integer was proved by Kűrschák in 1918 (Hoffman 1998, p. 157).
The harmonic numbers have odd numerators and even denominators. The th harmonic number is given asymptotically by
(13)

where is the EulerMascheroni constant (Conway and Guy 1996; Havil 2003, pp. 79 and 89), where the general th term is , giving , 120, , 240, ... for , 2, ... (OEIS A006953). This formula is a special case of an EulerMaclaurin integration formulas (Havil 2003, p. 79).
Inequalities bounding include
(14)

(Young 1991; Havil 2003, pp. 7375) and
(15)

(DeTemple 1991; Havil 2003, pp. 7678).
An interesting analytic sum is given by
(16)

(Coffman 1987). Borwein and Borwein (1995) show that
(17)
 
(18)
 
(19)
 
(20)
 
(21)

where is the Riemann zeta function. The first of these had been previously derived by de Doelder (1991), and the third by Goldbach in a 1742 letter to Euler (Borwein and Bailey 2003, pp. 99100; Bailey et al. 2007, p. 256). These identities are corollaries of the identity
(22)

(Borwein and Borwein 1995). Additional identities due to Euler are
(23)
 
(24)

for , 3, ... (Borwein and Borwein 1995), where is Apéry's constant. These sums are related to socalled Euler sums.
A general identity due to B. Cloitre (pers. comm., Jan. 7, 2006) is
(25)

where is a Pochhammer symbol.
Gosper gave the interesting identity
(26)
 
(27)

where is the incomplete gamma function and is the EulerMascheroni constant.
G. Huvent (2002) found the beautiful formula
(28)

A beautiful double series is given by
(29)

(Bailey et al. 2007, pp. 273274). Another double sum is
(30)

for (Sondow 2003, 2005).
There is an unexpected connection between the harmonic numbers and the Riemann hypothesis.
Generalized harmonic numbers in power can be defined by the relationship
(31)

where
(32)

These number are implemented as HarmonicNumber[n, r].
The numerators of the special case are known as Wolstenholme numbers. B. Cloitre (pers. comm., ) gave the surprising identity
(33)

which relates to an indefinite version of a famous series for . also satisfies
(34)

where is the Riemann zeta function. This follows from the identity
(35)

where is the trigamma function since
(36)

For odd , the generalized harmonic numbers have the explicit form
(37)

where is the polygamma function, is the gamma function, and is the Riemann zeta function.
The 2index harmonic numbers satisfy the identity
(38)

(P. Simon, pers. comm., Aug. 30, 2004).
Sums of the generalized harmonic numbers include
(39)

for , where is a polylogarithm,
(40)
 
(41)
 
(42)
 
(43)
 
(44)
 
(45)

where equations (40), (41), (42), and (44) are due to B. Cloitre (pers. comm., Oct. 4, 2004) and is a dilogarithm. In general,
(46)

(P. Simone, pers. comm. June 2, 2003). The power harmonic numbers also obey the unexpected identity
(47)

(M. Trott, pers. comm.).
P. Simone (pers. comm., Aug. 30, 2004) showed that
(48)

where
(49)
 
(50)
 
(51)
 
(52)

This gives the special results
(53)

for , respectively.
Conway and Guy (1996) define the secondorder harmonic number by
(54)
 
(55)
 
(56)

the thirdorder harmonic number by
(57)

and the thorder harmonic number by
(58)

A slightly different definition of a twoindex harmonic number is given by Roman (1992) in connection with the harmonic logarithm. Roman (1992) defines this by
(59)
 
(60)

plus the recurrence relation
(61)

For general and , this is equivalent to
(62)

and for , it simplifies to
(63)

For , the harmonic number can be written
(64)

where is the Roman factorial and is a Stirling number of the first kind.
A separate type of number sometimes also called a "harmonic number" is a harmonic divisor number (or Ore number).