# Harmonic progression (mathematics)

In mathematics, a **harmonic progression** (or **harmonic sequence**) is a progression formed by taking the reciprocals of an arithmetic progression.

Equivalently, a **sequence is a harmonic progression** when each term is the harmonic mean of the neighboring terms.

As a third equivalent characterization, it is an infinite sequence of the form

where *a* is not zero and −*a*/*d* is not a natural number, or a finite sequence of the form

where *a* is not zero, *k* is a natural number and −*a*/*d* is not a natural number or is greater than *k*.

## Examples[edit]

- 1, 1/2, 1/3, 1/4, 1/5, 1/6, sometimes referred to as the
*harmonic sequence* - 12, 6, 4, 3, , 2, … , , …
- 30, −30, −10, −6, − , … ,
- 10, 30, −30, −10, −6, − , … ,

## Sums of harmonic progressions[edit]

Infinite harmonic progressions are not summable (sum to infinity).

It is not possible for a harmonic progression of distinct unit fractions (other than the trivial case where *a* = 1 and *k* = 0) to sum to an integer. The reason is that, necessarily, at least one denominator of the progression will be divisible by a prime number that does not divide any other denominator.^{[1]}

## Use in geometry[edit]

If collinear points A, B, C, and D are such that D is the harmonic conjugate of C with respect to A and B, then the distances from any one of these points to the three remaining points form harmonic progression.^{[2]}^{[3]} Specifically, each of the sequences
AC, AB, AD; BC, BA, BD; CA, CD, CB; and DA, DC, DB are harmonic progressions, where each of the distances is signed according to a fixed orientation of the line.

In a triangle, if the altitudes are in arithmetic progression, then the sides are in harmonic progression.

## Leaning Tower of Lire[edit]

An excellent example of Harmonic Progression is the Leaning Tower of Lire. In it, uniform blocks are stacked on top of each other to achieve the maximum sideways or lateral distance covered. The blocks are stacked 1/2, 1/4, 1/6, 1/8, 1/10, … distance sideways below the original block. This ensures that the center of gravity is just at the center of the structure so that it does not collapse. A slight increase in weight on the structure causes it to become unstable and fall.

## See also[edit]

## References[edit]

**^**Erdős, P. (1932), "Egy Kürschák-féle elemi számelméleti tétel általánosítása" [Generalization of an elementary number-theoretic theorem of Kürschák] (PDF),*Mat. Fiz. Lapok*(in Hungarian),**39**: 17–24. As cited by Graham, Ronald L. (2013), "Paul Erdős and Egyptian fractions",*Erdős centennial*, Bolyai Soc. Math. Stud., vol. 25, János Bolyai Math. Soc., Budapest, pp. 289–309, CiteSeerX 10.1.1.300.91, doi:10.1007/978-3-642-39286-3_9, ISBN 978-3-642-39285-6, MR 3203600.**^***Chapters on the modern geometry of the point, line, and circle, Vol. II*by Richard Townsend (1865) p. 24**^***Modern geometry of the point, straight line, and circle: an elementary treatise*by John Alexander Third (1898) p. 44

*Mastering Technical Mathematics*by Stan Gibilisco, Norman H. Crowhurst, (2007) p. 221*Standard mathematical tables*by Chemical Rubber Company (1974) p. 102*Essentials of algebra for secondary schools*by Webster Wells (1897) p. 307