Page 1
Edurev123
4. Laurant's Series
4.1 Find the Laurent series of the function ?? (?? )=?????? ?[
?? ?? (?? -
?? ?? )] as ?
?? =-8
8
??? ?? ?? ?? for
?? <|?? |<8
where
?? ?? =
?? ?? ? ?
?? ?? ?????? ?(???? -?? ?????? ??? )???? ;?? =?? ,±?? ,±?? ,…..
with ?? a given complex number and taking the unit circle ?? given by ?? =?? ????
(-?? =
?? =?? ) as contour in this region.
(2010 : 15 Marks)
Solution:
?? is the only point of singularity here. So, we can expand exp?(
?? 2
(?? -
1
?? ))) as Laurent
series valid for every value of ?? ?0, i.e., for ???? >0.
Thus, we have
???????????????????????????????????????????????????????????????|?? |?0
??????????????????????????????????????exp?(
?? 2
(?? -
1
?? ))=?
?? =-8
8
??? ?? ?? ??
when
?? ?? =
1
2????
? ?
?? (?? )
?? 2
?? +1
2
?? +1
????
where ?? is any circle with its center at the origin. Taking ?? as the circle with radius unity,
we have
?? ?? =
1
2????
?
?? ?
?? ?? /2(?? -
1
?? )
?? ?? +1
????
Putting ?? =?? ????
, we obtain
Page 2
Edurev123
4. Laurant's Series
4.1 Find the Laurent series of the function ?? (?? )=?????? ?[
?? ?? (?? -
?? ?? )] as ?
?? =-8
8
??? ?? ?? ?? for
?? <|?? |<8
where
?? ?? =
?? ?? ? ?
?? ?? ?????? ?(???? -?? ?????? ??? )???? ;?? =?? ,±?? ,±?? ,…..
with ?? a given complex number and taking the unit circle ?? given by ?? =?? ????
(-?? =
?? =?? ) as contour in this region.
(2010 : 15 Marks)
Solution:
?? is the only point of singularity here. So, we can expand exp?(
?? 2
(?? -
1
?? ))) as Laurent
series valid for every value of ?? ?0, i.e., for ???? >0.
Thus, we have
???????????????????????????????????????????????????????????????|?? |?0
??????????????????????????????????????exp?(
?? 2
(?? -
1
?? ))=?
?? =-8
8
??? ?? ?? ??
when
?? ?? =
1
2????
? ?
?? (?? )
?? 2
?? +1
2
?? +1
????
where ?? is any circle with its center at the origin. Taking ?? as the circle with radius unity,
we have
?? ?? =
1
2????
?
?? ?
?? ?? /2(?? -
1
?? )
?? ?? +1
????
Putting ?? =?? ????
, we obtain
?? ?? ?=
1
2????
? ?
2?? 0
?
?? ?? 2
(?? ????
-?? -????
)
?? ?? (?? +1)?? ·???? ????
????
??????????????????????????????????????????????????????????????? ?? =
1
2?? ? ?
2?? 0
??? ???? sin??? ·?? -??????
????
????????????????????????????????????????????????? ?? =
1
2?? ?
0
2?? ?cos?(?? sin??? -???? )???? +
?? 2?? ?
0
2?? ?sin?(?? sin??? -???? )????
Taking ?? =2?? -?? , we have
?? ?
2?? 0
?sin?(?? sin??? -???? )???? -(-? ?
0
2?? ?sin?[-?? sin??? -2???? +???? ]???? )
?=? ?
2?? 0
?sin?(?? sin??? -???? )????
So that ?? ?
2?? 0
?sin?(?? sin??? -???? )???? =0
?????????????????????????????? ?? =
1
2?? ?
0
2?? ?cos?(?? sin??? -???? )???? +0 (Putting value from (2) in (1))
?=
1
2?? ? ?
2?? 0
?cos?(???? -?? sin??? )????
?=
2
2?? ? ?
?? 0
?cos?(???? -?? sin??? )????
?? ?? =
1
?? ? ?
?? 0
?cos?(???? -?? sin??? )????
So, given expression can be expressed as ?
-8
8
??? ?? ?? ?? where ?? ?? is given by above
equation (3).
4.2 Find the Laurent series for the function
?? (?? )=
?? ?? -?? ?? with center ?? =??
(2011: 15 marks)
Solution:
We have
?? (?? )?=
1
1-?? 2
=
1
(1-?? )(1+?? )
??????????????????????????????????????????????????????????????????=
1
?? (
1
1-?? +
1
1+?? )?????????????????????????????????????????????????????????????(??)
Page 3
Edurev123
4. Laurant's Series
4.1 Find the Laurent series of the function ?? (?? )=?????? ?[
?? ?? (?? -
?? ?? )] as ?
?? =-8
8
??? ?? ?? ?? for
?? <|?? |<8
where
?? ?? =
?? ?? ? ?
?? ?? ?????? ?(???? -?? ?????? ??? )???? ;?? =?? ,±?? ,±?? ,…..
with ?? a given complex number and taking the unit circle ?? given by ?? =?? ????
(-?? =
?? =?? ) as contour in this region.
(2010 : 15 Marks)
Solution:
?? is the only point of singularity here. So, we can expand exp?(
?? 2
(?? -
1
?? ))) as Laurent
series valid for every value of ?? ?0, i.e., for ???? >0.
Thus, we have
???????????????????????????????????????????????????????????????|?? |?0
??????????????????????????????????????exp?(
?? 2
(?? -
1
?? ))=?
?? =-8
8
??? ?? ?? ??
when
?? ?? =
1
2????
? ?
?? (?? )
?? 2
?? +1
2
?? +1
????
where ?? is any circle with its center at the origin. Taking ?? as the circle with radius unity,
we have
?? ?? =
1
2????
?
?? ?
?? ?? /2(?? -
1
?? )
?? ?? +1
????
Putting ?? =?? ????
, we obtain
?? ?? ?=
1
2????
? ?
2?? 0
?
?? ?? 2
(?? ????
-?? -????
)
?? ?? (?? +1)?? ·???? ????
????
??????????????????????????????????????????????????????????????? ?? =
1
2?? ? ?
2?? 0
??? ???? sin??? ·?? -??????
????
????????????????????????????????????????????????? ?? =
1
2?? ?
0
2?? ?cos?(?? sin??? -???? )???? +
?? 2?? ?
0
2?? ?sin?(?? sin??? -???? )????
Taking ?? =2?? -?? , we have
?? ?
2?? 0
?sin?(?? sin??? -???? )???? -(-? ?
0
2?? ?sin?[-?? sin??? -2???? +???? ]???? )
?=? ?
2?? 0
?sin?(?? sin??? -???? )????
So that ?? ?
2?? 0
?sin?(?? sin??? -???? )???? =0
?????????????????????????????? ?? =
1
2?? ?
0
2?? ?cos?(?? sin??? -???? )???? +0 (Putting value from (2) in (1))
?=
1
2?? ? ?
2?? 0
?cos?(???? -?? sin??? )????
?=
2
2?? ? ?
?? 0
?cos?(???? -?? sin??? )????
?? ?? =
1
?? ? ?
?? 0
?cos?(???? -?? sin??? )????
So, given expression can be expressed as ?
-8
8
??? ?? ?? ?? where ?? ?? is given by above
equation (3).
4.2 Find the Laurent series for the function
?? (?? )=
?? ?? -?? ?? with center ?? =??
(2011: 15 marks)
Solution:
We have
?? (?? )?=
1
1-?? 2
=
1
(1-?? )(1+?? )
??????????????????????????????????????????????????????????????????=
1
?? (
1
1-?? +
1
1+?? )?????????????????????????????????????????????????????????????(??)
First we will find Laurent expansion for ?? (?? )=
1
1+?? about ?? =1.
We write,
?? (?? )=??
8
?? =0
??? ?? (?? -1)
?? (???? )
where
?? ?? =
?? ?? (1)
?? !
But
?? ?? (?? )=(-1)(-2)……(-?? )(1+2)
-(?? +1)
=(-1)
?? ?? !(1+?? )
-(?? +1)
????????????????????????????????????????????????
?? ?? (1)
?? !
=
(-1)
?? 2
?? +1
????????????????????????????????????????????????????????? ?? =
?? ?? (1)
?? !
=
(-1)
?? 2
?? +1
From (ii),
?? (?? )=??
8
?? =0
(-1)
?? 2
?? +1
(?? -1)
??
=??
8
?? =0
(1-?? )
?? 2
?? +1
? From (i),
???????????????????????????????????????????????????????? (?? )=
1
2
(
1
1-?? +?
?? =0
8
?
(1-?? )
?? 2
?? +1
)
This is the required expansion.
4.3 Expand the function ?? (?? )=
?? (?? +?? )(?? +?? )
in Laurent series valid for :
(i) ?? <|?? |<?? ;
(ii) |?? |>?? ;
(iii) ?? <|?? +?? |<?? ;
(iv) |?? |<??
(2012 : 15 Marks)
Page 4
Edurev123
4. Laurant's Series
4.1 Find the Laurent series of the function ?? (?? )=?????? ?[
?? ?? (?? -
?? ?? )] as ?
?? =-8
8
??? ?? ?? ?? for
?? <|?? |<8
where
?? ?? =
?? ?? ? ?
?? ?? ?????? ?(???? -?? ?????? ??? )???? ;?? =?? ,±?? ,±?? ,…..
with ?? a given complex number and taking the unit circle ?? given by ?? =?? ????
(-?? =
?? =?? ) as contour in this region.
(2010 : 15 Marks)
Solution:
?? is the only point of singularity here. So, we can expand exp?(
?? 2
(?? -
1
?? ))) as Laurent
series valid for every value of ?? ?0, i.e., for ???? >0.
Thus, we have
???????????????????????????????????????????????????????????????|?? |?0
??????????????????????????????????????exp?(
?? 2
(?? -
1
?? ))=?
?? =-8
8
??? ?? ?? ??
when
?? ?? =
1
2????
? ?
?? (?? )
?? 2
?? +1
2
?? +1
????
where ?? is any circle with its center at the origin. Taking ?? as the circle with radius unity,
we have
?? ?? =
1
2????
?
?? ?
?? ?? /2(?? -
1
?? )
?? ?? +1
????
Putting ?? =?? ????
, we obtain
?? ?? ?=
1
2????
? ?
2?? 0
?
?? ?? 2
(?? ????
-?? -????
)
?? ?? (?? +1)?? ·???? ????
????
??????????????????????????????????????????????????????????????? ?? =
1
2?? ? ?
2?? 0
??? ???? sin??? ·?? -??????
????
????????????????????????????????????????????????? ?? =
1
2?? ?
0
2?? ?cos?(?? sin??? -???? )???? +
?? 2?? ?
0
2?? ?sin?(?? sin??? -???? )????
Taking ?? =2?? -?? , we have
?? ?
2?? 0
?sin?(?? sin??? -???? )???? -(-? ?
0
2?? ?sin?[-?? sin??? -2???? +???? ]???? )
?=? ?
2?? 0
?sin?(?? sin??? -???? )????
So that ?? ?
2?? 0
?sin?(?? sin??? -???? )???? =0
?????????????????????????????? ?? =
1
2?? ?
0
2?? ?cos?(?? sin??? -???? )???? +0 (Putting value from (2) in (1))
?=
1
2?? ? ?
2?? 0
?cos?(???? -?? sin??? )????
?=
2
2?? ? ?
?? 0
?cos?(???? -?? sin??? )????
?? ?? =
1
?? ? ?
?? 0
?cos?(???? -?? sin??? )????
So, given expression can be expressed as ?
-8
8
??? ?? ?? ?? where ?? ?? is given by above
equation (3).
4.2 Find the Laurent series for the function
?? (?? )=
?? ?? -?? ?? with center ?? =??
(2011: 15 marks)
Solution:
We have
?? (?? )?=
1
1-?? 2
=
1
(1-?? )(1+?? )
??????????????????????????????????????????????????????????????????=
1
?? (
1
1-?? +
1
1+?? )?????????????????????????????????????????????????????????????(??)
First we will find Laurent expansion for ?? (?? )=
1
1+?? about ?? =1.
We write,
?? (?? )=??
8
?? =0
??? ?? (?? -1)
?? (???? )
where
?? ?? =
?? ?? (1)
?? !
But
?? ?? (?? )=(-1)(-2)……(-?? )(1+2)
-(?? +1)
=(-1)
?? ?? !(1+?? )
-(?? +1)
????????????????????????????????????????????????
?? ?? (1)
?? !
=
(-1)
?? 2
?? +1
????????????????????????????????????????????????????????? ?? =
?? ?? (1)
?? !
=
(-1)
?? 2
?? +1
From (ii),
?? (?? )=??
8
?? =0
(-1)
?? 2
?? +1
(?? -1)
??
=??
8
?? =0
(1-?? )
?? 2
?? +1
? From (i),
???????????????????????????????????????????????????????? (?? )=
1
2
(
1
1-?? +?
?? =0
8
?
(1-?? )
?? 2
?? +1
)
This is the required expansion.
4.3 Expand the function ?? (?? )=
?? (?? +?? )(?? +?? )
in Laurent series valid for :
(i) ?? <|?? |<?? ;
(ii) |?? |>?? ;
(iii) ?? <|?? +?? |<?? ;
(iv) |?? |<??
(2012 : 15 Marks)
Solution:
Given:
?? (?? )?=
1
(?? +1)(?? +3)
?=
1
2(?? +1)
-
1
2(?? +3)
(using partial
fractions)
(i) ????????????????????????????????????????????????1<|?? |<3
????????????????????????????????????????
1
|?? |
?<1 and
|?? |
3
<1
??????????????????????????????????????? (?? )?=
1
2(?? +1)
-
1
2(?? +3)
?=
1
2?? (1+
1
?? )
-
1
6(1+
?? 3
)
?=
1
2?? (1+
1
?? )
-1
-
1
6
(1+
?? 3
)
-1
?=
1
2?? (1-
1
?? +
1
?? 2
-
1
?? 3
?·)-
1
6
(1-
?? 3
+
?? 2
9
-
?? 3
27
….)
?=?..+
1
2?? 3
-
1
2?? 2
+
1
2?? +
1
6
-
?? 18
+
?? 2
54
-
?? 3
162
…..
(ii) ?????????????????|?? |>3
?
3
|?? |
<1
??? (?? )=
1
2?? (1+
1
?? )
-1
-
1
2?? (1+
3
?? )
-1
=
1
2?? (1-
1
?? +
1
?? 2
-
1
?? 3
+?..)-
1
2?? (1-
3
?? +
9
?? 2
-
27
?? 3
…..)
=
1
?? 2
-
4
?? 3
+
13
?? 4
-
40
25
+?..
(iii)
0<|?? +1|<2
Put ?? +1=4
Then
Page 5
Edurev123
4. Laurant's Series
4.1 Find the Laurent series of the function ?? (?? )=?????? ?[
?? ?? (?? -
?? ?? )] as ?
?? =-8
8
??? ?? ?? ?? for
?? <|?? |<8
where
?? ?? =
?? ?? ? ?
?? ?? ?????? ?(???? -?? ?????? ??? )???? ;?? =?? ,±?? ,±?? ,…..
with ?? a given complex number and taking the unit circle ?? given by ?? =?? ????
(-?? =
?? =?? ) as contour in this region.
(2010 : 15 Marks)
Solution:
?? is the only point of singularity here. So, we can expand exp?(
?? 2
(?? -
1
?? ))) as Laurent
series valid for every value of ?? ?0, i.e., for ???? >0.
Thus, we have
???????????????????????????????????????????????????????????????|?? |?0
??????????????????????????????????????exp?(
?? 2
(?? -
1
?? ))=?
?? =-8
8
??? ?? ?? ??
when
?? ?? =
1
2????
? ?
?? (?? )
?? 2
?? +1
2
?? +1
????
where ?? is any circle with its center at the origin. Taking ?? as the circle with radius unity,
we have
?? ?? =
1
2????
?
?? ?
?? ?? /2(?? -
1
?? )
?? ?? +1
????
Putting ?? =?? ????
, we obtain
?? ?? ?=
1
2????
? ?
2?? 0
?
?? ?? 2
(?? ????
-?? -????
)
?? ?? (?? +1)?? ·???? ????
????
??????????????????????????????????????????????????????????????? ?? =
1
2?? ? ?
2?? 0
??? ???? sin??? ·?? -??????
????
????????????????????????????????????????????????? ?? =
1
2?? ?
0
2?? ?cos?(?? sin??? -???? )???? +
?? 2?? ?
0
2?? ?sin?(?? sin??? -???? )????
Taking ?? =2?? -?? , we have
?? ?
2?? 0
?sin?(?? sin??? -???? )???? -(-? ?
0
2?? ?sin?[-?? sin??? -2???? +???? ]???? )
?=? ?
2?? 0
?sin?(?? sin??? -???? )????
So that ?? ?
2?? 0
?sin?(?? sin??? -???? )???? =0
?????????????????????????????? ?? =
1
2?? ?
0
2?? ?cos?(?? sin??? -???? )???? +0 (Putting value from (2) in (1))
?=
1
2?? ? ?
2?? 0
?cos?(???? -?? sin??? )????
?=
2
2?? ? ?
?? 0
?cos?(???? -?? sin??? )????
?? ?? =
1
?? ? ?
?? 0
?cos?(???? -?? sin??? )????
So, given expression can be expressed as ?
-8
8
??? ?? ?? ?? where ?? ?? is given by above
equation (3).
4.2 Find the Laurent series for the function
?? (?? )=
?? ?? -?? ?? with center ?? =??
(2011: 15 marks)
Solution:
We have
?? (?? )?=
1
1-?? 2
=
1
(1-?? )(1+?? )
??????????????????????????????????????????????????????????????????=
1
?? (
1
1-?? +
1
1+?? )?????????????????????????????????????????????????????????????(??)
First we will find Laurent expansion for ?? (?? )=
1
1+?? about ?? =1.
We write,
?? (?? )=??
8
?? =0
??? ?? (?? -1)
?? (???? )
where
?? ?? =
?? ?? (1)
?? !
But
?? ?? (?? )=(-1)(-2)……(-?? )(1+2)
-(?? +1)
=(-1)
?? ?? !(1+?? )
-(?? +1)
????????????????????????????????????????????????
?? ?? (1)
?? !
=
(-1)
?? 2
?? +1
????????????????????????????????????????????????????????? ?? =
?? ?? (1)
?? !
=
(-1)
?? 2
?? +1
From (ii),
?? (?? )=??
8
?? =0
(-1)
?? 2
?? +1
(?? -1)
??
=??
8
?? =0
(1-?? )
?? 2
?? +1
? From (i),
???????????????????????????????????????????????????????? (?? )=
1
2
(
1
1-?? +?
?? =0
8
?
(1-?? )
?? 2
?? +1
)
This is the required expansion.
4.3 Expand the function ?? (?? )=
?? (?? +?? )(?? +?? )
in Laurent series valid for :
(i) ?? <|?? |<?? ;
(ii) |?? |>?? ;
(iii) ?? <|?? +?? |<?? ;
(iv) |?? |<??
(2012 : 15 Marks)
Solution:
Given:
?? (?? )?=
1
(?? +1)(?? +3)
?=
1
2(?? +1)
-
1
2(?? +3)
(using partial
fractions)
(i) ????????????????????????????????????????????????1<|?? |<3
????????????????????????????????????????
1
|?? |
?<1 and
|?? |
3
<1
??????????????????????????????????????? (?? )?=
1
2(?? +1)
-
1
2(?? +3)
?=
1
2?? (1+
1
?? )
-
1
6(1+
?? 3
)
?=
1
2?? (1+
1
?? )
-1
-
1
6
(1+
?? 3
)
-1
?=
1
2?? (1-
1
?? +
1
?? 2
-
1
?? 3
?·)-
1
6
(1-
?? 3
+
?? 2
9
-
?? 3
27
….)
?=?..+
1
2?? 3
-
1
2?? 2
+
1
2?? +
1
6
-
?? 18
+
?? 2
54
-
?? 3
162
…..
(ii) ?????????????????|?? |>3
?
3
|?? |
<1
??? (?? )=
1
2?? (1+
1
?? )
-1
-
1
2?? (1+
3
?? )
-1
=
1
2?? (1-
1
?? +
1
?? 2
-
1
?? 3
+?..)-
1
2?? (1-
3
?? +
9
?? 2
-
27
?? 3
…..)
=
1
?? 2
-
4
?? 3
+
13
?? 4
-
40
25
+?..
(iii)
0<|?? +1|<2
Put ?? +1=4
Then
0?<|?? +1|<2 P 0<|4|<2
?? (?? )?=
1
(?? +1)(?? +3)
=
1
4(4+2)
?=
1
21
(1+
4
2
)
-1
?=
1
24
(1-
4
2
+
4
2
4
-
4
3
8
….)
?=
1
24
-
1
4
+
4
8
-
4
2
16
+?.
?=
1
2(?? +1)
-
1
4
+
?? +1
8
-
(?? +1)
2
16
+?..
(iii) ???????????????????????|?? |<1
Then
?? (?? )?=
1
2(?? +1)
-
1
6(1+
?? 3
)
?=
1
2
(1-?? +?? 2
-?? 3
….)-
1
6
(1-
?? 3
+
?? 2
9
-
?? 3
27
…)
?=
1
3
-
4
9
?? +
13
17
?? 2
….
4.4 Expand in Laurent series the function ?? (?? )=
?? ?? ?? (?? -?? )
about ?? =?? and ?? =?? .
Solution:
Given that
?? (?? )=
1
?? 2
(?? -1)
About ?? =0, the Laurent Series is given by
1
?? 2
(?? -1)
?=
1
-?? 2
(1-?? )
=-
1
?? 2
(1-?? )
-1
?=-
1
?? 2
(1+?? +?? 2
+?? +?.)
?=-(
1
?? 2
+
1
?? +1+?? +?? 2
+?.)
Let ?? -1=?? ??? =?? +1 and
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