proof that Sylvester’s matrix equals the resultant

proof that Sylvester’s matrix equals the resultant

In the

derivation

of Sylvester’s matrix for the

resultant

, it was seen that if two

polynomials

have a common root, then Sylvester’s determinant will equal zero. Since two polynomials have a common root if and only if their resultant is zero, it follows that if the resultant is zero, then Sylvester’s determinant equals zero. In this entry, we shall use this fact to show that Sylvester’s determinant equals the resultant.

The secret is to view both Sylvester’s determinant and the resultant as

functions

of the roots. A more precise way of saying what this means is that we will study polnomials in the

indeterminates

a

0

,

r

1

,

r

2

,

…

,

r

m

,

b

0

,

s

1

,

s

2

,

…

,

s

n

. We will regard

a

1

,

a

2

,

…

,

a

m

,

b

1

,

b

2

,

…

,

b

m

as polynomials in these

variables

using the expression of

coefficients

of a polynomial as symmetric functions of its roots, e.g.

a

1

=

a

0

⁢

(

r

1

+

r

2

+

⋯

)

a

2

=

a

0

⁢

(

r

1

⁢

r

2

+

r

1

⁢

r

3

+

⋯

)

Note that

a

k

is a

k

th

order polynomial in the

r

i

’s and

b

k

is a

k

th

order polynomial in the

s

i

’s.

Let

R

be the polynomial

R

=

a

0

n

⁢

b

0

m

⁢

∏

i

=

1

m

∏

j

=

1

n

(

r

i

-

s

j

)

and let

D

be the polynomial which is gotten by replacing

occurrences

of

a

1

,

a

2

,

…

,

a

m

,

b

1

,

b

2

,

…

,

b

m

in Sylvester’s matrix by their expressions in of

a

0

,

r

1

,

r

2

,

…

,

r

m

,

b

0

,

s

1

,

s

2

,

…

,

s

n

. We want to show that

R

=

D

.

First, note that, in each row of Sylvester’s matrix, every entry is multiplied by either an

a

0

or a

b

0

. By a fundamental property of determinants, this means that we may pull all those factors of

a

0

and

b

0

outside the determinant. Since there are

n

rows containing

a

0

and

m

rows containing

b

0

, this means that

D

=

a

0

m

⁢

b

0

n

⁢

D

′

⁢

(

r

1

,

…

,

r

m

,

s

1

,

…

,

s

n

)

. Note that these factors correspond to the powers of

a

0

and

b

0

in the definition of

R

. Hence, to show that

D

=

R

it only remains to show that

D

′

=

R

′

, where

R

′

=

∏

i

=

1

m

∏

j

=

1

n

(

r

i

-

s

j

)

Second, note that the degree of

D

′

is not greater than the degree of

R

′

. From the definition, it is obvious that

R

′

is a polynomial of degree

m

⁢

n

. By examining Sylvester’s determinant and keeping in mind that

a

k

and

b

k

are of degree

k

, it is not hard to see that the degree of

D

′

cannot exceed

m

⁢

n

.

Third, we will show that

R

′

divides

D

′

. In the derivation of the Sylvester determinant, we saw that if

r

i

=

s

j

for some choice of

i

and

j

, then

D

=

0

, and hence

D

′

=

0

. The only way for a non-zero polynomial to to equal zero when

r

i

=

s

j

is for

r

i

-

s

i

to be a factor of the polynomial. It is easy to see that

D

′

is not the

zero polynomial

, and hence,

s

i

-

s

j

must be a factor of

D

′

. This means that every factor of

R

′

is also a factor of

D

′

. Since all the factors of

R

′

occur with

multiplicity

one, it follows that

D

′

is a

multiple

of

R

′

.

Combining the of the last two paragraphs, we come to the

conclusion

that

D

′

must be a

constant

multiple of

R

′

. To determine the constant of proportionality, all one needs to do is to compare the values of the two polynomials for a set of value of the variables for which they to not vanish. For

instance

, one could try

r

1

=

r

2

=

⋯

=

r

m

=

1

and

s

1

=

s

2

=

⋯

=

s

n

=

0

. Both

R

′

and

D

′

equal

1

for this special set of values, and hence

R

′

=

D

′

.

Title

proof that Sylvester’s matrix equals the resultant

Canonical name

ProofThatSylvestersMatrixEqualsTheResultant

Date of creation

2013-03-22 14:36:50

Last modified on

2013-03-22 14:36:50

Owner

rspuzio (6075)

Last modified by

rspuzio (6075)

Numerical id

8

Author

rspuzio (6075)

Entry type

Definition

Classification

msc 13P10

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