Real series/Introduction/Section
Let be a sequence of real numbers. The series is the sequence of the partial sums
If the sequence converges, then we say that the series converges. In this case, we write also
for its limit,
and this limit is called the sum of the series.All concepts for sequences carry over to series if we consider a series as the sequence of its partial sums . Like for sequences, it might happen that the sequence does not start with but later.
We want to compute the series
For this, we give a formula for the -th partial sum. We have
This sequence converges to , so that the series converges and its sum equals .
Let
denote convergent series of real numbers with sums and
respectively. Then the following statements hold.- The series given by is also convergent and its sum is .
- For also the series given by is convergent and its sum is .
Proof
Let
be a series of real numbers. Then the series is convergent if and only if the following Cauchy-criterion holds: For every there exists some such that for all
the estimate
Proof
It is therefore a necessary condition for the convergence of a series that its members form a null sequence. This condition is not sufficient, as the harmonic series shows.
The harmonic series is the series
This series diverges: For the numbers , we have
Therefore,
Hence, the sequence of the partial sums is unbounded, and so, due to fact, not convergent.
The following statement is called Leibniz criterion for alternating series.
Let be an decreasing null sequence of nonnegative real numbers. Then the series
converges.