# On the use of Laurent series

#### ognik

##### Active member
My book is a little confusing sometimes, and googling doesn't always help. Just a couple of queries - and please add any of your own 'tips & tricks'....

1) Laurent series (LS) is defined from $-\infty$, yet all the examples I have seen start from 0 - I can't think of an annulus with a negative radius myself, so do I just use it from 0 and not worry about the negative side of the domain?

2) What is the practical difference between a complex Taylor series (TS) and LS? I have seen suggested that TS is for holomorhpic functions and LS for isolated singularities, but it seems to me those conditions could apply to both TS & LS?

3) A difference I can see is that TS only allows for the region < disk radius, but LS provide for > some R (and also within an annulus) - so for an annulus could we use TS for inside the large radius, LS for outside the smaller radius?

4) For an annulus, couldn't we avoid using LS, Juts take TS of the outer - TS of the inner?

5) Conversely, could we use LS instead of TS, by making the smaller radius 0?

#### ZaidAlyafey

##### Well-known member
MHB Math Helper
My book is a little confusing sometimes, and googling doesn't always help. Just a couple of queries - and please add any of your own 'tips & tricks'....

1) Laurent series (LS) is defined from $-\infty$, yet all the examples I have seen start from 0 - I can't think of an annulus with a negative radius myself, so do I just use it from 0 and not worry about the negative side of the domain?
There is no relation between the negative radius and the index of the summation. Having a negative sign in the summation means that the function is not holomorphic at least in a circle of some radious.

2) What is the practical difference between a complex Taylor series (TS) and LS? I have seen suggested that TS is for holomorhpic functions and LS for isolated singularities, but it seems to me those conditions could apply to both TS & LS?
All TS are LS but not vice versa. As you said they are used to expand functions with singularities.

3) A difference I can see is that TS only allows for the region < disk radius, but LS provide for > some R (and also within an annulus) - so for an annulus could we use TS for inside the large radius, LS for outside the smaller radius?
By definition of TS it is used to expand the function around a point and the function has to be analytic on that point. So it has no meaning to say that the function has a TS on an annulus.

4) For an annulus, couldn't we avoid using LS, Juts take TS of the outer - TS of the inner?
If the function is holomorphic on an annulus then it has no TS in the inner radius.

5) Conversely, could we use LS instead of TS, by making the smaller radius 0?