# Exact bounds for complex roots of a polynomial: complexroot

### 5.29.2 Exact bounds for complex roots of a polynomial: complexroot

The complexroot command finds bounds for the complex roots of a polynomial.

- complexroot takes two mandatory arguments and two optional arguments:
  - P, a polynomial.
  - є, a postive real number.
  - Optionally, α, β, two complex numbers.
- complexroot(P,є) returns a list of vectors, where the elements of each vector are one of:
  - an interval (the boundaries of this interval are the opposite vertices of a rectangle with sides parallel to the axis and containing a complex root of the polynomial) and the multiplicity of this root.
    Suppose the interval is [a1+ib1,a2+ib2] then |a1−a2|<є, |b1−b2|<є and the root a+ib satisfies a1≤ a ≤ a2 and b1≤ b ≤ b2.
  - the value of an exact complex root of the polynomial and the multiplicity of this root.
- complexroot(P,є,α,β) returns a list of vectors as above, but only for the roots lying in the rectangle with sides parallel to the axis having α,β as opposite vertices.

#### Examples.

- Find the roots of x3+1.
  Input:
  complexroot(x^3+1,0.1)
  Output:

  ⎡
  ⎢
  ⎢
  ⎢
  ⎢
  ⎣

  −1 | 1

  ⎡ ⎣ | 0.499999046325680..0.500000953674320 | ⎤ ⎦ | − | ⎡ ⎣ | 0.866024494171135..0.866026401519779 | ⎤ ⎦ | i

  1

  ⎡ ⎣ | 0.499999046325680..0.500000953674320 | ⎤ ⎦ | + | ⎡ ⎣ | 0.866024494171135..0.866026401519779 | ⎤ ⎦ | i

  1

  ⎤
  ⎥
  ⎥
  ⎥
  ⎥
  ⎦

  Hence, for x3+1:
  - -1 is a root of multiplicity 1,
  - a+ib is a root of multiplicity 1 with 0.499999046325680 ≤ a ≤ 0.500000953674320 and −0.866026401519779 ≤ b ≤ −0.866024494171135.
  - c +id is a root of multiplicity 1 with 0.499999046325680 ≤ c ≤ 0.500000953674320 and 0.866024494171135 ≤ d ≤ 0.866026401519779.
- Find the roots of x3+1 lying inside the rectangle with opposite vertices −1,1+2*i.
  Input:
  complexroot(x^3+1,0.1,-1,1+2*i)
  Output:

  ⎡
  ⎢
  ⎢
  ⎣

  −1 | 1

  ⎡ ⎣ | 0.499999046325680..0.500000953674320 | ⎤ ⎦ | + | ⎡ ⎣ | 0.866024494171135..0.866026401519779 | ⎤ ⎦ | i

  1

  ⎤
  ⎥
  ⎥
  ⎦