# Arbitrary Precision Complex Intervals¶

Arbitrary Precision Complex Intervals

This is a simple complex interval package, using intervals which are axis-aligned rectangles in the complex plane. It has very few special functions, and it does not use any special tricks to keep the size of the intervals down.

AUTHORS:

These authors wrote complex_number.pyx:

• William Stein (2006-01-26): complete rewrite
• Joel B. Mohler (2006-12-16): naive rewrite into pyrex
• William Stein(2007-01): rewrite of Mohler’s rewrite

Then complex_number.pyx was copied to complex_interval.pyx and heavily modified:

• Carl Witty (2007-10-24): rewrite to become a complex interval package
• Travis Scrimshaw (2012-10-18): Added documentation to get full coverage.

Todo

Implement ComplexIntervalFieldElement multiplicative order similar to ComplexNumber multiplicative order with _set_multiplicative_order(n) and ComplexNumber.multiplicative_order() methods.

class sage.rings.complex_interval.ComplexIntervalFieldElement

A complex interval.

EXAMPLES:

sage: I = CIF.gen()
sage: b = 1.5 + 2.5*I
sage: TestSuite(b).run()

arg()

Same as argument().

EXAMPLES:

sage: i = CIF.0
sage: (i^2).arg()
3.141592653589794?

argument()

The argument (angle) of the complex number, normalized so that $$-\pi < \theta.lower() \leq \pi$$.

We raise a ValueError if the interval strictly contains 0, or if the interval contains only 0.

Warning

We do not always use the standard branch cut for argument! If the interval crosses the negative real axis, then the argument will be an interval whose lower bound is less than $$\pi$$ and whose upper bound is more than $$\pi$$; in effect, we move the branch cut away from the interval.

EXAMPLES:

sage: i = CIF.0
sage: (i^2).argument()
3.141592653589794?
sage: (1+i).argument()
0.785398163397449?
sage: i.argument()
1.570796326794897?
sage: (-i).argument()
-1.570796326794897?
sage: (RR('-0.001') - i).argument()
-1.571796326461564?
sage: CIF(2).argument()
0
sage: CIF(-2).argument()
3.141592653589794?


Here we see that if the interval crosses the negative real axis, then the argument can exceed $$\pi$$, and we we violate the standard interval guarantees in the process:

sage: CIF(-2, RIF(-0.1, 0.1)).argument().str(style='brackets')
'[3.0916342578678501 .. 3.1915510493117365]'
sage: CIF(-2, -0.1).argument()
-3.091634257867851?

bisection()

Returns the bisection of self into four intervals whose union is self and intersection is center().

EXAMPLES:

sage: z = CIF(RIF(2, 3), RIF(-5, -4))
sage: z.bisection()
(3.? - 5.?*I, 3.? - 5.?*I, 3.? - 5.?*I, 3.? - 5.?*I)
sage: for z in z.bisection():
...       print z.real().endpoints(), z.imag().endpoints()
(2.00000000000000, 2.50000000000000) (-5.00000000000000, -4.50000000000000)
(2.50000000000000, 3.00000000000000) (-5.00000000000000, -4.50000000000000)
(2.00000000000000, 2.50000000000000) (-4.50000000000000, -4.00000000000000)
(2.50000000000000, 3.00000000000000) (-4.50000000000000, -4.00000000000000)

sage: z = CIF(RIF(sqrt(2), sqrt(3)), RIF(e, pi))
sage: a, b, c, d = z.bisection()
sage: a.intersection(b).intersection(c).intersection(d) == CIF(z.center())
True

sage: zz = a.union(b).union(c).union(c)
sage: zz.real().endpoints() == z.real().endpoints()
True
sage: zz.imag().endpoints() == z.imag().endpoints()
True

center()

Returns the closest floating-point approximation to the center of the interval.

EXAMPLES:

sage: CIF(RIF(1, 2), RIF(3, 4)).center()
1.50000000000000 + 3.50000000000000*I

conjugate()

Return the complex conjugate of this complex number.

EXAMPLES:

sage: i = CIF.0
sage: (1+i).conjugate()
1 - 1*I

contains_zero()

Returns True if self is an interval containing zero.

EXAMPLES:

sage: CIF(0).contains_zero()
True
sage: CIF(RIF(-1, 1), 1).contains_zero()
False

crosses_log_branch_cut()

Returns True if this interval crosses the standard branch cut for log() (and hence for exponentiation) and for argument. (Recall that this branch cut is infinitesimally below the negative portion of the real axis.)

EXAMPLES:

sage: z = CIF(1.5, 2.5) - CIF(0, 2.50000000000000001); z
1.5000000000000000? + -1.?e-15*I
sage: z.crosses_log_branch_cut()
False
sage: CIF(-2, RIF(-0.1, 0.1)).crosses_log_branch_cut()
True

diameter()

Returns a somewhat-arbitrarily defined “diameter” for this interval.

The diameter of an interval is the maximum of the diameter of the real and imaginary components, where diameter on a real interval is defined as absolute diameter if the interval contains zero, and relative diameter otherwise.

EXAMPLES:

sage: CIF(RIF(-1, 1), RIF(13, 17)).diameter()
2.00000000000000
sage: CIF(RIF(-0.1, 0.1), RIF(13, 17)).diameter()
0.266666666666667
sage: CIF(RIF(-1, 1), 15).diameter()
2.00000000000000

exp()

Compute $$e^z$$ or $$\exp(z)$$ where $$z$$ is the complex number self.

EXAMPLES:

sage: i = ComplexIntervalField(300).0
sage: z = 1 + i
sage: z.exp()
1.46869393991588515713896759732660426132695673662900872279767567631093696585951213872272450? + 2.28735528717884239120817190670050180895558625666835568093865811410364716018934540926734485?*I

imag()

Return imaginary part of self.

EXAMPLES:

sage: i = ComplexIntervalField(100).0
sage: z = 2 + 3*i
sage: x = z.imag(); x
3
sage: x.parent()
Real Interval Field with 100 bits of precision

intersection(other)

Returns the intersection of the two complex intervals self and other.

EXAMPLES:

sage: CIF(RIF(1, 3), RIF(1, 3)).intersection(CIF(RIF(2, 4), RIF(2, 4))).str(style='brackets')
'[2.0000000000000000 .. 3.0000000000000000] + [2.0000000000000000 .. 3.0000000000000000]*I'
sage: CIF(RIF(1, 2), RIF(1, 3)).intersection(CIF(RIF(3, 4), RIF(2, 4)))
Traceback (most recent call last):
...
ValueError: intersection of non-overlapping intervals

is_exact()

Returns whether this complex interval is exact (i.e. contains exactly one complex value).

EXAMPLES:

sage: CIF(3).is_exact()
True
sage: CIF(0, 2).is_exact()
True
sage: CIF(-4, 0).sqrt().is_exact()
True
sage: CIF(-5, 0).sqrt().is_exact()
False
sage: CIF(0, 2*pi).is_exact()
False
sage: CIF(e).is_exact()
False
sage: CIF(1e100).is_exact()
True
sage: (CIF(1e100) + 1).is_exact()
False

is_square()

This function always returns True as $$\CC$$ is algebraically closed.

EXAMPLES:

sage: CIF(2, 1).is_square()
True

log(base=None)

Complex logarithm of $$z$$.

Warning

This does always not use the standard branch cut for complex log! See the docstring for argument() to see what we do instead.

EXAMPLES:

sage: a = CIF(RIF(3, 4), RIF(13, 14))
sage: a.log().str(style='brackets')
'[2.5908917751460420 .. 2.6782931373360067] + [1.2722973952087170 .. 1.3597029935721503]*I'
sage: a.log().exp().str(style='brackets')
'[2.7954667135098274 .. 4.2819545928390213] + [12.751682453911920 .. 14.237018048974635]*I'
sage: a in a.log().exp()
True


If the interval crosses the negative real axis, then we don’t use the standard branch cut (and we violate the interval guarantees):

sage: CIF(-3, RIF(-1/4, 1/4)).log().str(style='brackets')
'[1.0986122886681095 .. 1.1020725100903968] + [3.0584514217013518 .. 3.2247338854782349]*I'
sage: CIF(-3, -1/4).log()
1.102072510090397? - 3.058451421701352?*I


Usually if an interval contains zero, we raise an exception:

sage: CIF(RIF(-1,1),RIF(-1,1)).log()
Traceback (most recent call last):
...
ValueError: Can't take the argument of interval strictly containing zero


But we allow the exact input zero:

sage: CIF(0).log()
[-infinity .. -infinity]


If a base is passed from another function, we can accommodate this:

sage: CIF(-1,1).log(2)
0.500000000000000? + 3.399270106370396?*I

norm()

Returns the norm of this complex number.

If $$c = a + bi$$ is a complex number, then the norm of $$c$$ is defined as the product of $$c$$ and its complex conjugate:

$ext{norm}(c) = ext{norm}(a + bi) = c \cdot \overline{c} = a^2 + b^2.$

The norm of a complex number is different from its absolute value. The absolute value of a complex number is defined to be the square root of its norm. A typical use of the complex norm is in the integral domain $$\ZZ[i]$$ of Gaussian integers, where the norm of each Gaussian integer $$c = a + bi$$ is defined as its complex norm.

EXAMPLES:

sage: CIF(2, 1).norm()
5
sage: CIF(1, -2).norm()
5

overlaps(other)

Returns True if self and other are intervals with at least one value in common.

EXAMPLES:

sage: CIF(0).overlaps(CIF(RIF(0, 1), RIF(-1, 0)))
True
sage: CIF(1).overlaps(CIF(1, 1))
False

plot(pointsize=10, **kwds)

Plot a complex interval as a rectangle.

EXAMPLES:

sage: sum(plot(CIF(RIF(1/k, 1/k), RIF(-k, k))) for k in [1..10])


Exact and nearly exact points are still visible:

sage: plot(CIF(pi, 1), color='red') + plot(CIF(1, e), color='purple') + plot(CIF(-1, -1))


A demonstration that $$z \mapsto z^2$$ acts chaotically on $$|z|=1$$:

sage: z = CIF(0, 2*pi/1000).exp()
sage: g = Graphics()
sage: for i in range(40):
...       z = z^2
...       g += z.plot(color=(1./(40-i), 0, 1))
...
sage: g

prec()

Return precision of this complex number.

EXAMPLES:

sage: i = ComplexIntervalField(2000).0
sage: i.prec()
2000

real()

Return real part of self.

EXAMPLES:

sage: i = ComplexIntervalField(100).0
sage: z = 2 + 3*i
sage: x = z.real(); x
2
sage: x.parent()
Real Interval Field with 100 bits of precision

sqrt(all=False, **kwds)

The square root function.

Warning

We approximate the standard branch cut along the negative real axis, with sqrt(-r^2) = i*r for positive real r; but if the interval crosses the negative real axis, we pick the root with positive imaginary component for the entire interval.

INPUT:

• all – bool (default: False); if True, return a list of all square roots.

EXAMPLES:

sage: CIF(-1).sqrt()^2
-1
sage: sqrt(CIF(2))
1.414213562373095?
sage: sqrt(CIF(-1))
1*I
sage: sqrt(CIF(2-I))^2
2.00000000000000? - 1.00000000000000?*I
sage: CC(-2-I).sqrt()^2
-2.00000000000000 - 1.00000000000000*I


Here, we select a non-principal root for part of the interval, and violate the standard interval guarantees:

sage: CIF(-5, RIF(-1, 1)).sqrt().str(style='brackets')
'[-0.22250788030178321 .. 0.22250788030178296] + [2.2251857651053086 .. 2.2581008643532262]*I'
sage: CIF(-5, -1).sqrt()
0.222507880301783? - 2.247111425095870?*I

str(base=10, style=None)

Returns a string representation of self.

EXAMPLES:

sage: CIF(1.5).str()
'1.5000000000000000?'
sage: CIF(1.5, 2.5).str()
'1.5000000000000000? + 2.5000000000000000?*I'
sage: CIF(1.5, -2.5).str()
'1.5000000000000000? - 2.5000000000000000?*I'
sage: CIF(0, -2.5).str()
'-2.5000000000000000?*I'
sage: CIF(1.5).str(base=3)
'1.1111111111111111111111111111111112?'
sage: CIF(1, pi).str(style='brackets')
'[1.0000000000000000 .. 1.0000000000000000] + [3.1415926535897931 .. 3.1415926535897936]*I'


• RealIntervalFieldElement.str()
union(other)

Returns the smallest complex interval including the two complex intervals self and other.

EXAMPLES:

sage: CIF(0).union(CIF(5, 5)).str(style='brackets')
'[0.00000000000000000 .. 5.0000000000000000] + [0.00000000000000000 .. 5.0000000000000000]*I'


Return the complex number defined by the strings s_real and s_imag as an element of ComplexIntervalField(prec=n), where $$n$$ potentially has slightly more (controlled by pad) bits than given by $$s$$.

INPUT:

• s_real – a string that defines a real number (or something whose string representation defines a number)
• s_imag – a string that defines a real number (or something whose string representation defines a number)
• pad – an integer at least 0.
• min_prec – number will have at least this many bits of precision, no matter what.

EXAMPLES:

sage: ComplexIntervalFieldElement('2.3')
2.300000000000000?
sage: ComplexIntervalFieldElement('2.3','1.1')
2.300000000000000? + 1.100000000000000?*I
sage: ComplexIntervalFieldElement(10)
10
sage: ComplexIntervalFieldElement(10,10)
10 + 10*I
sage: ComplexIntervalFieldElement(1.000000000000000000000000000,2)
1 + 2*I
sage: ComplexIntervalFieldElement(1,2.000000000000000000000)
1 + 2*I
sage: ComplexIntervalFieldElement(1.234567890123456789012345, 5.4321098654321987654321)
1.234567890123456789012350? + 5.432109865432198765432000?*I


TESTS:

Make sure we’ve rounded up log(10,2) enough to guarantee sufficient precision (trac ticket #10164). This is a little tricky because at the time of writing, we don’t support intervals long enough to trip the error. However, at least we can make sure that we either do it correctly or fail noisily:

sage: c_CIFE = sage.rings.complex_interval.create_ComplexIntervalFieldElement
sage: for kp in range(2,6):
...       s = '1.' + '0'*10**kp + '1'
...       try:
...           assert c_CIFE(s,0).real()-1 != 0
...           assert c_CIFE(0,s).imag()-1 != 0
...       except TypeError:
...           pass

sage.rings.complex_interval.is_ComplexIntervalFieldElement(x)

Check if x is a ComplexIntervalFieldElement.

EXAMPLES:

sage: from sage.rings.complex_interval import is_ComplexIntervalFieldElement as is_CIFE
sage: is_CIFE(CIF(2))
True
sage: is_CIFE(CC(2))
False

sage.rings.complex_interval.make_ComplexIntervalFieldElement0(fld, re, im)

Construct a ComplexIntervalFieldElement for pickling.

TESTS:

sage: a = CIF(1 + I)
sage: loads(dumps(a)) == a # indirect doctest
True


#### Previous topic

Field of Arbitrary Precision Complex Intervals