# Harmonic sequence

In algebra, a harmonic sequence, sometimes called a harmonic progression, is a sequence of numbers such that the difference between the reciprocals of any two consecutive terms is constant. In other words, a harmonic sequence is formed by taking the reciprocals of every term in an arithmetic sequence.

For example, $-1, -\frac{1}{2}, -\frac{1}{3}, -\frac{1}{4}$ and $6, 3, 2, \frac{6}{4}$ are harmonic sequences; however, $0, \frac{1}{3}, \frac{1}{6}, \frac{1}{9}$ and $\frac{1}{4}, \frac{1}{12}, \frac{1}{36}, \frac{1}{108}, \ldots$ are not.

More formally, a harmonic progression $a_1, a_2, \ldots , a_n$ biconditionally satisfies $1/a_2 - 1/a_1 = 1/a_3 - 1/a_2 = \cdots = 1/a_n - 1/a_{n-1}.$ A similar definition holds for infinite harmonic sequences. It appears most frequently in its three-term form: namely, that constants $a$, $b$, and $c$ are in harmonic progression if and only if $1/b - 1/a = 1/c - 1/b$.

## Properties

Because the reciprocals of the terms in a harmonic sequence are in arithmetic progression, one can apply properties of arithmetic sequences to derive a general form for harmonic sequences. Namely, for some constants $a$ and $d$, the terms of any finite harmonic sequence can be written as $$\frac{1}{a}, \textrm{ } \frac{1}{a+d}, \textrm{ } \frac{1}{a+2d},\textrm{ } \cdots \textrm{ }, \frac{1}{a+(n-1)d}.$$

A common lemma is that a sequence is in harmonic progression if and only if $a_n$ is the harmonic mean of $a_{n-1}$ and $a_{n+1}$ for any consecutive terms $a_{n-1}, a_n, a_{n+1}$. In symbols, $2/a_n = 1/a_{n-1} + 1/a_{n+1}$. This is mostly used to perform substitutions, though it occasionally serves as a definition of harmonic sequences.

## Sum

A harmonic series is the sum of all the terms in a harmonic series. All infinite harmonic series diverges, which follows by the limit comparison test with the series $1 + 1/2 + 1/3 + \cdots$. This series is referred to as the harmonic series. As for finite harmonic series, there is no known general expression for their sum; one must find a strategy to evaluate one on a case-by-case basis.

## Examples

Here are some example problems that utilize harmonic sequences and series.

### Example 1

Find all real numbers such that $x+4, x+1, x$ is a harmonic sequence.

Solution: Using the harmonic mean properties of harmonic sequences, $$\frac{2}{x+1} = \frac{1}{x+4} + \frac{1}{x} = \frac{2x+4}{x(x+4)}.$$ Note that $x=-4, -1, 0$ would create a term of $0$—something that breaks the definition of harmonic sequences—which eliminates them as possible solutions. We can thus multiply both sides by $x(x+1)(x+4)$ to get $2x(x+4) = (2x+4)(x+1)$. Expanding these factors yields $2x^2 + 8x = 2x^2 + 6x + 4$, which simplifies to $x=2$. Thus, $2, 3, 6$ is the only solution to the equation, as desired. $\square$

### Example 2

Let $a$, $b$, and $c$ be positive real numbers. Show that if $a$, $b$, and $c$ are in harmonic progression, then $a/(b+c)$, $b/(c+a)$, and $c/(a+b)$ are as well.

Solution: Using the harmonic mean property of harmonic sequences, we are given that $1/a + 1/b = 2/c$, and we wish to show that $(b+c)/a + (a+b)/c = 2(c+a)/b$. We work backwards from the latter equation.

One approach might be to add $2$ to both sides of the equation, which when combined with the fractions returns $$\frac{a+b+c}{a} + \frac{a+b+c}{c} = \frac{2(a+b+c)}{b}.$$ Because $a$, $b$, and $c$ are all positive, $a+b+c \neq 0$. Thus, we can divide both sides of the equation by $a+b+c$ to get $1/a + 1/c = 2/b$, which was given as true.

From here, it is easy to write the proof forwards. Doing so proves that $(b+c)/a + (a+b)/c = 2(c+a)/b$, which implies that $a/(b+c)$, $b/(c+a)$, $c/(a+b)$ is a harmonic sequence, as required. $\square$

### Example 3

2019 AMC 10A Problem 15: A sequence of numbers is defined recursively by $a_1 = 1$, $a_2 = \frac{3}{7}$, and $a_n = \frac{a_{n-2} \cdot a_{n-1}}{2 a_{n-2} - a_{n-1}}$ for all $n \geq 3$ Then $a_{2019}$ can be written as $\frac{p}{q}$, where $p$ and $q$ are relatively prime positive integers. What is $p+q$?

Solution: We simplify the series' recursive formula. Taking the reciprocals of both sides, we get the equality $$\frac{1}{a_n} = \frac{2 a_{n-2} - a_{n-1}}{a_{n-1} \cdot a_{n-2}} = \frac{2}{a_{n-2}} - \frac{1}{a_{n-1}}.$$ Thus, $2/a_{n_1} = 1/a_{n-2} + 1/a_n$. This is the harmonic mean, which implies that $a_{n-1}, a_n, a_{n+1}$ is a harmonic progression. Thus, the entire sequence is in harmonic progression.

Using the tools of harmonic sequences, we will now find a closed expression for the sequence. Let $a_1 = 1/a$ and $a_2 = 1/(a+d)$. Simplifying the first equation yields $a=1$ and substituting this into the second equation yields $d = 4/3$. Thus, $$a_n = \frac{1}{1 + \frac{4}{3}(n-1)},$$ and so $a_{2019} = 8075 / 3$. The answer is then $8075 + 3 = 8078$. $\square$

## More Problems

Here are some more problems that utilize harmonic sequences and series. Note that harmonic sequences are rather uncommon compared to their arithmetic and geometric counterparts.