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.
  1. The series given by is also convergent and its sum is .
  2. 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

holds.

Proof



Let

denote a convergent series of real numbers. Then

This follows directly from fact.


Nikolaus of Oresme (1330-1382) proved that the harmonic series diverges.

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

So this series is about the "infinite sum“ of the unit fractions

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 divergence of the harmonic series implies that one can construct with equal building bricks an arbitrary large overhang.

The following statement is called Leibniz criterion for alternating series.


Let be an decreasing null sequence of nonnegative real numbers. Then the series

converges.

Proof

This proof was not presented in the lecture.