Infinite series, the sum of infinitely many numbers related in a given way and listed in a given order. Infinite series are useful in mathematics and in such disciplines as physics, chemistry, biology, and engineering.

For an infinite series a1 + a2 + a3 +⋯, a quantity sn = a1 + a2 +⋯+ an, which involves adding only the first n terms, is called a partial sum of the series. If sn approaches a fixed number S as n becomes larger and larger, the series is said to converge. In this case, S is called the sum of the series. An infinite series that does not converge is said to diverge. In the case of divergence, no value of a sum is assigned. For example, the nth partial sum of the infinite series 1 + 1 + 1 +⋯ is n. As more terms are added, the partial sum fails to approach any finite value (it grows without bound). Thus, the series diverges. An example of a convergent series is

As n becomes larger, the partial sum approaches 2, which is the sum of this infinite series. In fact, the series 1 + r + r2 + r3 +⋯ (in the example above r equals 1/2) converges to the sum 1/(1 − r) if 0 < r < 1 and diverges if r ≥ 1. This series is called the geometric series with ratio r and was one of the first infinite series to be studied. Its solution goes back to Zeno of Elea’s paradox involving a race between Achilles and a tortoise (see mathematics, foundations of: Being versus becoming).

Certain standard tests can be applied to determine the convergence or divergence of a given series, but such a determination is not always possible. In general, if the series a1 + a2 +⋯ converges, then it must be true that an approaches 0 as n becomes larger. Furthermore, adding or deleting a finite number of terms from a series never affects whether or not the series converges. Furthermore, if all the terms in a series are positive, its partial sums will increase, either approaching a finite quantity (converging) or growing without bound (diverging). This observation leads to what is called the comparison test: if 0 ≤ an ≤ bn for all n and if b1 + b2 +⋯ is a convergent infinite series, then a1 + a2 +⋯ also converges. When the comparison test is applied to a geometric series, it is reformulated slightly and called the ratio test: if an > 0 and if an + 1/an ≤ r for some r < 1 for every n, then a1 + a2 +⋯ converges. For example, the ratio test proves the convergence of the series

Many mathematical problems that involve a complicated function can be solved directly and easily when the function can be expressed as an infinite series involving trigonometric functions (sine and cosine). The process of breaking up a rather arbitrary function into an infinite trigonometric series is called Fourier analysis or harmonic analysis and has numerous applications in the study of various wave phenomena.