Root test


In mathematics, the root test is a criterion for the convergence of an infinite series. It depends on the quantity
where are the terms of the series, and states that the series converges absolutely if this quantity is less than one, but diverges if it is greater than one. It is particularly useful in connection with power series.

Root test explanation

The root test was developed first by Augustin-Louis Cauchy who published it in his textbook Cours d'analyse. Thus, it is sometimes known as the Cauchy root test or Cauchy's radical test. For a series
the root test uses the number
where "lim sup" denotes the limit superior, possibly ∞+. Note that if
converges then it equals C and may be used in the root test instead.
The root test states that:
There are some series for which C = 1 and the series converges, e.g., and there are others for which C = 1 and the series diverges, e.g..

Application to power series

This test can be used with a power series
where the coefficients cn, and the center p are complex numbers and the argument z is a complex variable.
The terms of this series would then be given by an = cnn. One then applies the root test to the an as above. Note that sometimes a series like this is called a power series "around p", because the radius of convergence is the radius R of the largest interval or disc centred at p such that the series will converge for all points z strictly in the interior. A corollary of the root test applied to such a power series is the Cauchy–Hadamard theorem: the radius of convergence is exactly taking care that we really mean ∞ if the denominator is 0.

Proof

The proof of the convergence of a series Σan is an application of the comparison test. If for all n ≥ N we have then. Since the geometric series converges so does by the comparison test. Hence Σan converges absolutely.
If for infinitely many n, then an fails to converge to 0, hence the series is divergent.
Proof of corollary:
For a power series Σan = Σcnn, we see by the above that the series converges if there exists an N such that for all n ≥ N we have
equivalent to
for all n ≥ N, which implies that in order for the series to converge we must have for all sufficiently large n. This is equivalent to saying
so Now the only other place where convergence is possible is when
and this will not change the radius of convergence since these are just the points lying on the boundary of the interval or disc, so