Ask Your Question

# Partial fraction decomposition over the reals or complex

Hi,

I am trying to get partial fraction decomposition (for integration) but over the complex numbers. For example, the denominator (x^2+1)*(x^3-2) should produce five fractions. I have experimented a bit with coercion but didn't manage to get anything. For example:

f=x^3-2
R=CC['x']
ff=R(f)
factor(f); factor(ff)


produces

x^3 - 2
(x - 1.25992104989487) * (x + 0.629960524947437 - 1.09112363597172*I) * (x + 0.629960524947437 + 1.09112363597172*I)


so far so good (except for decimals). But (1/f).partial_fraction() produces 1/(x^3 - 2), and (1/ff).partial_fraction() produces

Traceback (click to the left of this block for traceback)
...
AttributeError: 'FractionFieldElement_1poly_field' object has no attribute 'partial_fraction'


On the other hand, (1/ff).partial_fraction_decomposition() gives

(0, [0.209986841649145/(x - 1.25992104989487), (-0.104993420824573 +
0.181853939328620*I)/(x + 0.629960524947437 - 1.09112363597172*I),
(-0.104993420824573 - 0.181853939328620*I)/(x + 0.629960524947437 +
1.09112363597172*I)])


which is correct but not exact. Is there a command to get an exact decomposition over C, or am I stuck with having to set and solve a linear system?

Thanks

edit retag close merge delete

## Comments

I don't know how to do the computation like you ask, but I can tell you why it's not exact: you're working over CC, so you're asking for a numerical answer.

( 2012-11-21 20:12:01 -0600 )edit

## 1 answer

Sort by ยป oldest newest most voted

Expanding on Snark's comment: use algebraic numbers to get an exact answer.

Algebraic numbers can be explored, you can ask their minimal polynomial or a radical expression.

sage: R.<x> = QQbar['x']
sage: ff = x^3 - 2
sage: factor(ff)
(x - 1.259921049894873?) * (x + 0.6299605249474365? - 1.091123635971722?*I) * (x + 0.6299605249474365? + 1.091123635971722?*I)
sage: (1/ff).partial_fraction_decomposition()
(0,
[(0.2099868416491456? + 0.?e-19*I)/(x - 1.259921049894873?),
(-0.10499342082457277? + 0.1818539393286203?*I)/(x + 0.6299605249474365? - 1.091123635971722?*I),
(-0.10499342082457277? - 0.1818539393286203?*I)/(x + 0.6299605249474365? + 1.091123635971722?*I)])


The entries are exact; they live in QQbar. To investigate the entries:

sage: for q in factor(ff):
....:     q = q[0]
....:     print 'x - a =', q
....:     a = QQbar(x - q)
....:     print '  a =', a
....:     print '  minpoly:', a.minpoly()
....:     print '  radical:', a.radical_expression()
....:
x - a = x - 1.259921049894873?
a = 1.259921049894873?
minpoly: x^3 - 2
radical: 2^(1/3)
x - a = x + 0.6299605249474365? - 1.091123635971722?*I
a = -0.6299605249474365? + 1.091123635971722?*I
minpoly: x^3 - 2
radical: 1/2*I*sqrt(3)*2^(1/3) - 1/2*2^(1/3)
x - a = x + 0.6299605249474365? + 1.091123635971722?*I
a = -0.6299605249474365? - 1.091123635971722?*I
minpoly: x^3 - 2
radical: -1/2*I*sqrt(3)*2^(1/3) - 1/2*2^(1/3)


and

sage: for q in (1/ff).partial_fraction_decomposition()[1]:
....:     print 'a / (x - b) =', q
....:     a = QQbar(q.numerator())
....:     print '  a =', a
....:     print '  minpoly:', a.minpoly()
....:     print '  radical:', a.radical_expression()
....:     b = QQbar(x - q.denominator())
....:     print '  b =', b
....:     print '  minpoly:', b.minpoly()
....:     print '  radical:', b.radical_expression()
....:
a / (x - b) = 0.2099868416491456?/(x - 1.259921049894873?)
a = 0.2099868416491456?
minpoly: x^3 - 1/108
radical: 1/12*4^(2/3)
b = 1.259921049894873?
minpoly: x^3 - 2
radical: 2^(1/3)
a / (x - b) = (-0.10499342082457277? + 0.1818539393286203?*I)/(x + 0.6299605249474365? - 1.091123635971722?*I)
a = -0.10499342082457277? + 0.1818539393286203?*I
minpoly: x^3 - 1/108
radical: 1/24*4^(2/3)*(I*sqrt(3) - 1)
b = -0.6299605249474365? + 1.091123635971722?*I
minpoly: x^3 - 2
radical: 1/2*I*sqrt(3)*2^(1/3) - 1/2*2^(1/3)
a / (x - b) = (-0.10499342082457277? - 0.1818539393286203?*I)/(x + 0.6299605249474365? + 1.091123635971722?*I)
a = -0.10499342082457277? - 0.1818539393286203?*I
minpoly: x^3 - 1/108
radical: 1/24*4^(2/3)*(-I*sqrt(3) - 1)
b = -0.6299605249474365? - 1.091123635971722?*I
minpoly: x^3 - 2
radical: -1/2*I*sqrt(3)*2^(1/3) - 1/2*2^(1/3)

more

## Comments

But this does not answer how you get it to print out an exact answer. That is, I want it to print out: (x-2^(1/3)), ( x + (2^(1.3) )/2 + i ( 2^(1/3)* (sqrt(3)/2) ), ( x + (2^(1.3) )/2 - i ( 2^(1/3)* (sqrt(3)/2) )

How do I make it print this out?

( 2017-06-03 20:16:47 -0600 )edit

I edited my answer to show how to print radical expressions for all the algebraic numbers involved.

( 2017-09-01 12:24:23 -0600 )edit

## Your Answer

Please start posting anonymously - your entry will be published after you log in or create a new account.

Add Answer

## Stats

Asked: 2012-11-20 08:29:04 -0600

Seen: 3,023 times

Last updated: Sep 01 '17