Hyperharmonic number

In mathematics, the n-th hyperharmonic number of order r, denoted by , is recursively defined by the relations:

and

In particular, is the n-th harmonic number.

The hyperharmonic numbers were discussed by J. H. Conway and R. K. Guy in their 1995 book The Book of Numbers.[1]:258

Identities involving hyperharmonic numbers

By definition, the hyperharmonic numbers satisfy the recurrence relation

In place of the recurrences, there is a more effective formula to calculate these numbers:

The hyperharmonic numbers have a strong relation to combinatorics of permutations. The generalization of the identity

reads as

where is an r-Stirling number of the first kind.[2]

Asymptotics

The above expression with binomial coefficients easily gives that for all fixed order r>=2 we have.[3]

that is, the quotient of the left and right hand side tends to 1 as n tends to infinity.

An immediate consequence is that

when m>r.

Generating function and infinite series

The generating function of the hyperharmonic numbers is

The exponential generating function is much more harder to deduce. One has that for all r=1,2,...

where 2F2 is a hypergeometric function. The r=1 case for the harmonic numbers is a classical result, the general one was proved in 2009 by I. Mező and A. Dil.[4]

The next relation connects the hyperharmonic numbers to the Hurwitz zeta function:[3]

An open conjecture

It is known, that the harmonic numbers are never integers except the case n=1. The same question can be posed with respect to the hyperharmonic numbers: are there integer hyperharmonic numbers? István Mező proved[5] that if r=2 or r=3, these numbers are never integers except the trivial case when n=1. He conjectured that this is always the case, namely, the hyperharmonic numbers of order r are never integers except when n=1. This conjecture was justified for a class of parameters by R. Amrane and H. Belbachir.[6] Especially, these authors proved that is not integer for all n=2,3,...

External links

Notes

  1. John H., Conway; Richard K., Guy (1995). The book of numbers. Copernicus.
  2. Benjamin, A. T.; Gaebler, D.; Gaebler, R. (2003). "A combinatorial approach to hyperharmonic numbers". Integers (3): 1–9.
  3. 1 2 Mező, István; Dil, Ayhan (2010). "Hyperharmonic series involving Hurwitz zeta function". Journal of Number Theory. 130: 360–369. doi:10.1016/j.jnt.2009.08.005.
  4. Mező, István; Dil, Ayhan (2009). "Euler-Seidel method for certain combinatorial numbers and a new characterization of Fibonacci sequence". Central European Journal of Mathematics. 7 (2): 310–321. doi:10.2478/s11533-009-0008-5.
  5. Mező, István (2007). "About the non-integer property of the hyperharmonic numbers". Annales Universitatis Scientarium Budapestinensis de Rolando Eötvös Nominatae, Sectio Mathematica (50): 13–20.
  6. Amrane, R. A.; Belbachir, H. (2010). "Non-integerness of class of hyperharmonic numbers". Annales Mathematicae et Informaticae (37): 7–11.
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