p-Series



A series such as

1 + 1/4 + 1/9
+ 1/16 + ... = 1/(1^2) + 1/(2^2) + 1/(3^2) + 1/(4^2) + ... = the sum over
n from 1 to infinity of 1/(n^2)

is called a p-series. In general, a p-series follows the following form:

the sum over n from
1 to infinity of 1/(n^p) = 1/(1^p) + 1/(2^p) + 1/(3^p) + ...

p-series are useful because of the following theorem:

The p-series

the sum over n from 1
to infinity of 1/(n^p)

is convergent if p > 1 and divergent otherwise.

Unfortunately, there is no simple theorem to give us the sum of a p-series. For instance, the sum of the example series is

1 + 1/4 + 1/9
+ ... = the sum over n from 1 to infinity of 1/(n^2) = (pi^2)/6.

If p=1, we call the resulting series the harmonic series:

1 + 1/2 + 1/3 +
... = the sum over n from 1 to infinity of 1/n

By the above theorem, the harmonic series does not converge.



Copyright © 1996 Department of Mathematics, Oregon State University

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