# Changes

,  06:01, 22 March 2015
Split introduction into simple and advanced topics, added syntax highlighting, use decorated intervals per default
=== Input and output ===
Before exercising interval arithmetic, interval objects must be created from non-interval data. There are interval constants <code>empty</code> and <code>entire</code> and the class interval constructors <code>infsupinfsupdec</code> (create an interval from boundaries), <code>midrad</code> for bare intervals (create an interval from midpoint and radius) and <code>infsupdechull</code> (create an interval enclosure for decorated a list of mixed arguments: numbers, intervalsor interval literals). The class constructors are very sophisticated and can be used with several kinds of parameters: Interval boundaries can be given by numeric values or string values with decimal numbers. Also it is possible to use so called interval literals with square brackets.
{{Code|Create intervals for performing interval arithmetic|<syntaxhighlight lang="octave:1"> infsup infsupdec (1) ans = # [1]_com octave:2> infsup infsupdec (1, 2) ans = # [1, 2]_com octave:3> infsup infsupdec ("3", "4") ans = # [3, 4]_com octave:4> infsup infsupdec ("1.1") ans ⊂ # [1.0999999999999998, 1.1000000000000001]_com octave:infsupdec ("5> infsup (.8e-17") # [5.799999999999999e-17, 65.5800000000000001e-17]_commidrad (12, 3) # [9, 15]_commidrad ("4.2", "1e-7") ans = # [54.199999899999999, 64.52000001000000005]_com octave:6> infsup hull (3, 42, "19.3", "-2.3") # [5-2.8e-173000000000000003, +42]_trvhull ("pi", "e") ans ⊂ # [52.799999999999999e-17718281828459045, 53.800000000000001e-171415926535897936]_trv</syntaxhighlight>}}
It is possible to access the exact numeric interval boundaries with the functions <code>inf</code> and <code>sup</code>. The shown default text representation of intervals can be created with <code>intervaltotext</code>. The default text representation is not guaranteed to be exact (see function <code>intervaltoexact</code> for that purpose), because this would massively spam console output. For example, the exact text representation of <code>realmin</code> would be over 700 decimal places long! However, the default text representation is correct as it guarantees to contain the actual boundaries and is accurate enough to separate different boundaries.
octave:7> infsup {{Warning|Decimal fractions as well as numbers of high magnitude (1, 1 + eps) ans ⊂ [1, 1.0000000000000003] octave:8> infsup (1, 1 + 2 * eps<sup>53</sup>) ans ⊂ [1should always be passed as a string to the constructor. Otherwise it is possible, 1that GNU Octave introduces conversion errors when the numeric literal is converted into floating-point format '''before''' it is passed to the constructor.0000000000000005]}}
Warning: Decimal fractions as well as numbers {{Code|Beware of the conversion pitfall|<syntaxhighlight lang="octave">## The numeric constant “0.2” is an approximation of high magnitude the## decimal number 0.2. An interval around this approximation## will not contain the decimal number 0.2.infsupdec (> 0.2<sup>53</sup>) should always be passed # [.20000000000000001, .20000000000000002]_com## However, passing the decimal number 0.2 as a string ## to the interval constructorwill create an interval which## actually encloses the decimal number. Otherwise it is possible, that GNU Octave introduces conversion errors when the numeric literal is converted into floating-point format '''before''' it is passed to the constructorinfsupdec ("0. In simple cases it may help to construct intervals with the 2") # [https://www.gnu19999999999999998, .org20000000000000002]_com</software/octave/doc/interpreter/Commands.html#Commands command syntax].syntaxhighlight>}}
octave:9> infsup (==== Interval vectors and matrices ====Vectors and matrices of intervals can be created by passing numerical matrices, char vectors or cell arrays to the interval constructors. With cell arrays it is also possible to mix several types of boundaries. Interval matrices behave like normal matrices in GNU Octave and can be used for broadcasting and vectorized function evaluation. {{Code|Create interval matrices|<span style syntaxhighlight lang= "color:redoctave">0.M = infsup (magic (3)) # [8] [1] [6] # [3] [5] [7] # [4] [9] [2</span>]infsup (magic (3), magic (3) + 1) ans ⊂ # [8, 9] [1, 2] [6, 7] # [3, 4] [5, 6] [.200000000000000017, .200000000000000028] octave: # [4, 5] [9, 10> infsup ] [2, 3]infsupdec (<span style = ["color:green0.1">; "0.2"</span>; "0.3"; "0.4"; "0.5"]) ans ⊂ # [.09999999999999999, .10000000000000001]_com # [.19999999999999998, .20000000000000002]_com # [.29999999999999998, .30000000000000005]_com # [.39999999999999996, .40000000000000003]_com # [.5]_com octave:11> infsup <span style = ({1, eps; "4/7", "pi"}, {2, 1; "e", "color:green0xff">0}) # [1, 2] [2.220446049250313e-16, 1] # [.5714285714285713, 2.7182818284590456] [3.1415926535897931, 255]</spansyntaxhighlight> ans ⊂ }} === Arithmetic operations ===The interval packages comprises many interval arithmetic operations. A complete list can be found in its [http://octave.sourceforge.19999999999999998net/interval/overview.html function reference]. Function names match GNU Octave standard functions where applicable and follow recommendations by IEEE 1788 otherwise, cf.20000000000000002[[#IEEE_1788_index|IEEE 1788 index]].
For convenience it Arithmetic functions in a set-based interval arithmetic follow these rules: Intervals are sets. They are subsets of the set of real numbers. The interval version of an elementary function such as sin(''x'') is possible essentially the natural extension to implicitly call sets of the corresponding point-wise function on real numbers. That is, the function is evaluated for each number in the interval constructor during all interval operations if at least one input already where the function is defined and the result must be an interval objectenclosure of all possible values that may occur.
octave:12> infsup By default arithmetic functions are computed with best possible accuracy ("17.7"which is more than what is guaranteed by GNU Octave core functions) + 1 ans ⊂ [18.699999999999999, 18.700000000000003] octave:13> ans + "[0, 2]" ans ⊂ [18The result will therefore be a tight and very accurate enclosure of the true mathematical value in most cases.699999999999999Details on each function's accuracy can be found in its documentation, 20which is accessible with GNU Octave's help command.700000000000003]
==== Specialized {{Code|Examples of using interval constructors ===arithmetic functions|<syntaxhighlight lang="octave">Above mentioned interval construction with decimal numbers or numeric data is straightforwardsin (infsupdec (0.5)) # [. Beyond that47942553860420294, there are more ways to define intervals or interval boundaries.47942553860420301]_com* Hexadecimal-floating-constant form: Each interval boundary may be defined by a hexadecimal number pow (infsupdec (optionally containing a point2) and an exponent field with an integral power of two as defined by the C99 standard , infsupdec (3, 4)) # [http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf ISO/IEC98998, N125616]_comatan2 (infsupdec (1), §6infsupdec (1)) # [.4.47853981633974482, .27853981633974484]_commidrad (magic (3). This can be used as a convenient way to define interval boundaries in double-precision, because the hexadecimal form is much shorter than the decimal representation of many numbers0.5) * pascal (3)* Rational literals: Each interval boundary may be defined as a fraction of two decimal numbers # [13. This is especially useful if interval boundaries shall be tightest enclosures of fractions5, that would be hard to write down as a decimal number16.5]_com [25, 31]_com [42, 52]_com* Uncertain form: The interval as a whole can be defined by a midpoint or upper/lower boundary and an integral number of # [http://en13.wikipedia5, 16.org/wiki/Unit_in_the_last_place “units in last place” (ULPs)5]_com [31, 37]_com [55, 65] as an uncertainty_com # [13. The format is <code>''m''?''ruE''</code>5, where** <code>''m ''</code> is a mantissa in decimal16.5]_com [25,** <code>''r ''</code> is either empty (which means ½ ULP) or is a non-negative decimal integral ULP count or is the <code>?</code> character (for unbounded intervals)31]_com [38,48]_com** <code>''u ''</code> is either empty (symmetrical uncertainty of ''r'' ULPs in both directions) or is either <code>u</code> (up) or <code>d</codesyntaxhighlight> (down),** <code>''E ''</code> is either empty or an exponent field comprising the character <code>e</code> followed by a decimal integer exponent (base 10).}}
octave:14> infsup === Numerical operations ===Some operations on intervals do not return an interval enclosure, but a single number ("0x1.999999999999Apin double-4"precision) ans ⊂ [.1, .10000000000000001] octave:15Most important are <code> infsup ("1inf</3", "7code> and <code>sup</9") ans ⊂ [.33333333333333331, .7777777777777778] octave:16code> infsup ("121.2?") ans ⊂ [121.14999999999999, 121which return the lower and upper interval boundaries.25] octave:17> infsup ("5?32e2") ans = [-2700, +3700] octave:18> infsup ("-42??u") ans = [-42, +Inf]
==== Interval vectors and matrices ====Vectors and matrices More such operations are <code>mid</code> (approximation of intervals can be created by passing numerical matricesthe interval's midpoint), char vectors or cell arrays to the <code>infsupwid</code> constructor. With cell arrays it is also possible to mix several types (approximation of boundaries. octave:19> M = infsup (magic (3)) M = 3×3 the interval matrix [8] [1] [6] [3] [5] [7] [4] [9] [2] octave:20> infsup (magic (3's width), magic <code>rad</code> (3approximation of the interval's radius) + 1) ans = 3×3 interval matrix [8, 9] [1, 2] [6, 7] [3, 4] [5, 6] [7, 8] [4, 5] [9, 10] [2, 3] octave:21<code>mag</code> infsup (["0.1"; "0.2"; "0.3"; "0.4"; "0.5"]interval's magnitude) ans ⊂ 5×1 interval vector [.09999999999999999, .10000000000000001] [.19999999999999998, .20000000000000002] [.29999999999999998, .30000000000000005] [.39999999999999996, .40000000000000003] [.5] octave:22and <code>mig</code> infsup ({1, eps; "4/7", "pi"}, {2, 1; "e", "0xff"}interval's mignitude) ans ⊂ 2×2 interval matrix [1, 2] [2.220446049250313e-16, 1] [.5714285714285713, 2.7182818284590456] [3.1415926535897931, 255]
When matrices === Boolean operations ===Interval comparison operations produce boolean results. While some comparisons are resized using subscripted assignmentespecially for intervals (subset, interior, any implicit new matrix elements will carry an empty intervalismember, isempty, disjoint, …) others are extensions of simple numerical comparison. octave:23For example, the less-or-equal comparison is mathematically defined as ∀<sub>''a''</sub> ∃<sub>''b''</sub> ''a'' ≤ ''b'' ∧ ∀<sub>''b''</sub> ∃<sub>''a''</sub> M (4, 4) = 42 M = 4×4 interval matrix [8] [1] [6] [Empty] [3] [5] [7] [Empty] [4] [9] [2] [Empty] [Empty] [Empty] [Empty] [42] ''a'' ≤ ''b''.
Note <span style="opacity: Whilst most functions (<code.5">size</code>, <codeoctave:1>isvector</code>, <code>ismatrix</codespan>infsup (1, 3) work as expected on interval data types, the function <code>'''isempty'''</code> is evaluated element-wise and checks if an interval equals the empty set. octave:24> builtin = infsup ("isempty"2, empty ()4), isempty (empty ()) ans = 0
ans = 1

=== Matrix operations ===
Above mentioned operations can also be applied element-wise to interval vectors and matrices. Many operations use [http://www.gnu.org/software/octave/doc/interpreter/Vectorization-and-Faster-Code-Execution.html#Vectorization-and-Faster-Code-Execution vectorization techniques].

In addition, there are matrix operations on interval matrices. These operations comprise: dot product, matrix multiplication, vector sums (all with tightest accuracy), matrix inversion, matrix powers, and solving linear systems (the latter are less accurate). As a result of missing hardware / low-level library support and missing optimizations, these operations are relatively slow compared to familiar operations in floating-point arithmetic.

{{Code|Examples of using interval matrix functions|<syntaxhighlight lang="octave">
A = infsup ([1, 2, 3; 4, 0, 0; 0, 0, 1]); A (2, 3) = "[0, 6]"
# [1] [2] [3]
# [4] [0] [0, 6]
# [0] [0] [1]
B = inv (A)
# [0] [.25] [-1.5, 0]
# [.5] [-.125] [-1.5, -.75]
# [0] [0] [1]
A * B
# [1] [0] [-1.5, +1.5]
# [0] [1] [-6, +6]
# [0] [0] [1]

A = infsup (magic (3))
# [8] [1] [6]
# [3] [5] [7]
# [4] [9] [2]
c = A \ [3; 4; 5]
# [.18333333333333326, .18333333333333349]
# [.43333333333333329, .43333333333333341]
# [.18333333333333315, .18333333333333338]
A * c
# [2.9999999999999982, 3.0000000000000018]
# [3.9999999999999982, 4.0000000000000018]
# [4.9999999999999982, 5.0000000000000018]
</syntaxhighlight>
}}

==== Notes on linear systems ====
A system of linear equations in the form A''x'' = b with intervals can be seen as a range of ''classical'' linear systems, which can be solved simultaneously. Whereas classical algorithms compute an approximation for a single solution of a single linear system, interval algorithms compute an enclosure for all possible solutions of (possibly several) linear systems. Some characteristics should definitely be known when linear interval systems are solved:
* If the linear system is underdetermined and has infinitely many solutions, the interval solution will be unbound in at least one of its coordinates. Contrariwise, from an unbound result it can not be concluded whether the linear system is underdetermined or has solutions.
* If the interval result is empty in at least one of its coordinates, the linear system is guaranteed to be underdetermined and has no solutions. Contrariwise, from a non-empty result it can not be concluded whether all or some of the systems have solutions or not.
* Wide intervals within the matrix A can easily lead to a superposition of cases, where the rank of A is no longer unique. If the linear interval system contains cases of linear independent equations as well as linear dependent equations, the resulting enclosure of solutions will inevitably be very broad.

However, solving linear systems with interval arithmetic can produce useful results in many cases and automatically carries a guaranty for error boundaries. Additionally, it can give better information than the floating-point variants for some cases.

{{Code|Standard floating point arithmetic versus interval arithmetic on ill-conditioned linear systems|<syntaxhighlight lang="octave">
A = [1, 0; 2, 0];
## This linear system has no solutions
A \ [3; 0]
# warning: matrix singular to machine precision, rcond = 0
# 0.60000
# 0.00000
## This linear system has many solutions
A \ [4; 8]
# 4
# 0

## The empty interval vector proves that there is no solution
infsup (A) \ [3; 0]
# [Empty]
# [Empty]
## The unbound interval vector indicates that there may be many solutions
infsup (A) \ [4; 8]
# [4]
# [Entire]
</syntaxhighlight>
}}

=== Error handling ===

Due to the nature of set-based interval arithmetic, one should not observe errors (in the sense of raised GNU Octave error messages) during computation unless operations are evaluated for incompatible data types. Arithmetic operations which are not defined for (parts of) their input, simply ignore anything that is outside of their domain.

However, the interval constructors can produce errors depending on the input. The <code>infsup</code> constructor will fail if the interval boundaries are invalid. Contrariwise, the (preferred) <code>infsupdec</code>, <code>midrad</code> and <code>hull</code> constructors will only issue a warning and return a [NaI] object, which will propagate and survive through computations. NaI stands for “not an interval”.

{{Code|Effects of set-based interval arithmetic on partial functions and the NaI object|<syntaxhighlight lang="octave">
## Evaluation of a function outside of its domain returns an empty interval
infsupdec (2) / 0 # [Empty]_trv
infsupdec (0) ^ infsupdec (0) # [Empty]_trv

## Illegal interval construction creates a NaI
infsupdec (3, 2) # [NaI]
## NaI even survives through computations
ans + 1 # [NaI]
</syntaxhighlight>
}}
=== Decorations ===
With the subclass <code>infsupdec</code> it is possible to extend The interval arithmetic with package provides a powerful decoration systemfor intervals, as specified by the IEEE standard for interval arithmetic. Every By default any interval and intermediate result will additionally carry carries a decoration, which may provide collects additional information about the final resultcourse of function evaluation on the interval data.
Although Only the examples in this wiki are mostly presented with (unfavored) <code>infsup</code> constructor creates bare, undecorated intervals (for simplicity)and the <code>intervalpart</code> operation may be used to demote decorated intervals into bare, it undecorated ones. It is highly recommended to always use the decorated interval arithmetic by default, which gives additional information about an interval result in exchange for a tiny overhead.
The following decorations are available:
|}
In the following example, all The decoration information is lost especially useful after a very long and complicated function evaluation. For example, when the interval “def” decoration survives until the final result, it is possibly divided by zero, i. e., proven that the overall function is not guaranteed to be actually defined for all possible inputsvalues covered by the input intervals.
{{Code|Examples of using the decoration system|<syntaxhighlight lang="octave:1"> x = infsupdec (3, 4) ans = # [3, 4]_com octave:2> ans + 12 ans y = x - 3.5 # [15-.5, 16+.5]_com octave:3> ans / "## The square root function ignores any negative part of the input,## but the decoration indicates whether this has or has not happened.sqrt (x) # [01.7320508075688771, 2]"_com ans = sqrt (y) # [70, .5, Inf7071067811865476]_trv</syntaxhighlight>}}
=== Arithmetic operations Specialized interval constructors ===The Above mentioned interval packages comprises many construction with decimal numbers or numeric data is straightforward. Beyond that, there are more ways to define intervals or interval arithmetic operationsboundaries. Function names match GNU Octave * Hexadecimal-floating-constant form: Each interval boundary may be defined by a hexadecimal number (optionally containing a point) and an exponent field with an integral power of two as defined by the C99 standard functions where applicable([http://www.open-std.org/jtc1/sc22/WG14/www/docs/n1256.pdf ISO/IEC9899, and follow recommendations by IEEE 1788 otherwiseN1256, cf§6.4.4. [[#IEEE_1788_index|IEEE 1788 index]2])Arithmetic functions This can be used as a convenient way to define interval boundaries in a setdouble-based precision, because the hexadecimal form is much shorter than the decimal representation of many numbers.* Rational literals: Each interval arithmetic follow these rules: Intervals are setsboundary may be defined as a fraction of two decimal numbers. They are subsets This is especially useful if interval boundaries shall be tightest enclosures of the set of real numbersfractions, that would be hard to write down as a decimal number. * Uncertain form: The interval version as a whole can be defined by a midpoint or upper/lower boundary and an integral number of [http://en.wikipedia.org/wiki/Unit_in_the_last_place “units in last place” (ULPs)] as an elementary function such as sin(uncertainty. The format is <code>''m''?''ruE''</code>, where** <code>''m ''</code> is a mantissa in decimal,** <code>''xr ''</code> is either empty (which means ½ ULP) or is essentially the natural extension to sets of the corresponding pointa non-wise function on real numbers. That negative decimal integral ULP count or isthe <code>?</code> character (for unbounded intervals), the function ** <code>''u ''</code> is evaluated for each number either empty (symmetrical uncertainty of ''r'' ULPs in the interval where the function both directions) or is either <code>u</code> (up) or <code>d</code> (down),** <code>''E ''</code> is defined and either empty or an exponent field comprising the result must be an enclosure of all possible values that may occurcharacter <code>e</code> followed by a decimal integer exponent (base 10).
{{Code|Examples of different formats during interval construction|<syntaxhighlight lang="octave:1"> sin (infsup (0"0x1.5)999999999999Ap-4")# hex-form ans ⊂ # [.479425538604202941, .4794255386042030110000000000000001] octave:2> pow (infsup (2"1/3", "7/9")# rational form # [.33333333333333331, .7777777777777778]infsup (3, 4)"121.2?")# uncertain form ans = # [8121.14999999999999, 16121.25] octave:3> atan2 (infsup (1"5?32e2")# uncertain form with ulp count # [-2700, +3700]infsup (1)"-42??u")# unbound uncertain form ans ⊂ # [.7853981633974482-42, .7853981633974484+Inf]</syntaxhighlight>}}
=== Reverse arithmetic operations ===
[[File:Reverse-power-functions.png|400px|thumb|right|Reverse power operations. A relevant subset of the function's domain is outlined and hatched. In this example we use ''x''<sup>''y''</sup> ∈ [2, 3].]]
Some arithmetic functions also provide reverse mode operations. That is inverse functions with interval constraints. For example the <code>sqrrev</code> can compute the inverse of the <code>sqr</code> function on intervals. The syntax is <code>sqrrev sqrrev (C, X)</code> and will compute the enclosure of all numbers ''x'' ∈ X ∈ X that fulfill the constraint ''x''² ∈ C² ∈ C.
In the following example, we compute the constraints for base and exponent of the power function <code>pow</code> as shown in the figure.
<span style="opacity:.5">octave:1> </span> x = powrev1 (infsup ("[1.1, 1.45]"), infsup (2, 3))
x ⊂ [1.6128979635153646, 2.7148547265657915]
<span style="opacity:.5">octave:2> </span> y = powrev2 (infsup ("[2.14, 2.5]"), infsup (2, 3))
y ⊂ [.7564707973660299, 1.4440113978403284]
=== Numerical operations Tips & Tricks ===Some For convenience it is possible to implicitly call the interval constructor during all interval operations on intervals do not return if at least one input already is an interval enclosure, but a single number (in double-precision). Most important are <code>inf</code> and <code>sup</code>, which return the lower and upper interval boundariesobject.
More such operations are <codespan style="opacity:.5">octave:1>mid</codespan> infsupdec (approximation of the interval's midpoint"17.7")+ 1 ans ⊂ [18.699999999999999, 18.700000000000003]_com <codespan style="opacity:.5">octave:2>wid</codespan> (approximation of the interval's width)ans + "[0, <code>rad</code> (approximation of the interval's radius)2]" ans ⊂ [18.699999999999999, <code>mag</code> and <code>mig</code>20.700000000000003]_com
=== Boolean operations ===Interval comparison operations produce boolean resultsfunctions with only one argument can be called by using property syntax, e. While some comparisons are especially for intervals (subset, interior, ismember, isempty, disjoint, …) others are extensions of simple numerical comparison g. For example, the less-or-equal comparison is mathematically defined as ∀<subcode>''a''x.inf</subcode> ∃, <subcode>''b''x.sup</subcode> ''a'' ≤ ''b'' ∧ ∀or even <subcode>''b''</sub> ∃<sub>''a''x.sqr</subcode> ''a'' ≤ ''b''.
octave:1> infsup (1, 3) <= infsup (2, 4) ans = 1 === Matrix operations ===Above mentioned operations can also be applied element-wise to interval vectors and When matrices. Many operations use [http://www.gnu.org/software/octave/doc/interpreter/Vectorization-and-Faster-Code-Execution.html#Vectorization-and-Faster-Code-Execution vectorization techniques]. In additionare resized using subscripted assignment, there are any implicit new matrix operations on elements will carry an empty interval matrices. These operations comprise <span style="opacity: dot product, matrix multiplication, vector sums (all with tightest accuracy), matrix inversion, matrix powers, and solving linear systems (the latter are less accurate). As a result of missing hardware / low-level library support and missing optimizations, these operations are relatively slow compared to familiar operations in floating-point arithmetic.  5">octave:1> A </span>M = infsup ([1, 2, magic (3; 4, 0, 0; 0, 0, 1])); A M (2, 3) = "[04, 6]" A = 3×3 interval matrix [1] [2] [3] [4] [0] [0, 6] [0] [0] [1] octave:2> B = inv (A) B = 3×3 interval matrix [0] [.25] [-1.5, 0] [.5] [-.125] [-1.5, -.75] [0] [0] [1]42 octave:3> A * B ans M = 3×3 4×4 interval matrix [1] [0] [-1.5, +1.5] [0] [1] [-6, +6] [0] [0] [1]
[8] [1] [6] [Empty]
[3] [5] [7] [Empty]
[4] [9] [2] [Empty]
[Empty] [Empty] [Empty] [42]
octave:4> A = infsup Whilst most functions (magic (3)) A = 3×3 interval matrix [8] [1] [6] [3] [5] [7] [4] [9] [2] octave:5> c = A \ [3; 4; 5] c ⊂ 3×1 interval vector [.18333333333333326, .18333333333333349] [.43333333333333329, .43333333333333341] [.18333333333333315, .18333333333333338] octave:6> A * c ans ⊂ 3×1 interval vector [2.9999999999999982, 3.0000000000000018] [3.9999999999999982, 4.0000000000000018] [4.9999999999999982, 5.0000000000000018] ==== Notes on linear systems ====A system of linear equations in the form A''x'' = b with intervals can be seen as a range of ''classical'' linear systems, which can be solved simultaneously. Whereas classical algorithms compute an approximation for a single solution of a single linear system, interval algorithms compute an enclosure for all possible solutions of (possibly several) linear systems. Some characteristics should definitely be known when linear interval systems are solved:* If the linear system is underdetermined and has infinitely many solutions, the interval solution will be unbound in at least one of its coordinates. Contrariwise, from an unbound result it can not be concluded whether the linear system is underdetermined or has solutions.* If the interval result is empty in at least one of its coordinates, the linear system is guaranteed to be underdetermined and has no solutions. Contrariwise, from a non-empty result it can not be concluded whether all or some of the systems have solutions or not.* Wide intervals within the matrix A can easily lead to a superposition of cases, where the rank of A is no longer unique. If the linear interval system contains cases of linear independent equations as well as linear dependent equations, the resulting enclosure of solutions will inevitably be very broad. However, solving linear systems with interval arithmetic can produce useful results in many cases and automatically carries a guaranty for error boundaries. Additionally, it can give better information than the floating-point variants for some cases. {| class="wikitable" style="margin: auto"!Standard floating point arithmetic!Interval arithmetic|-| style = "vertical-align: top" | <span style="opacity:.5">octave:1code> size</spancode>A = [1, 0; 2, 0]; <span style="opacity:.5">octave:2code> isvector</spancode>A \ [3; 0] # no solution warning: matrix singular to machine precision, rcond = 0 ans = 0.60000 0.00000 <span style="opacity:.5">octave:3code> ismatrix</spancode>A \ [4; 8] # many solutions ans = 4 0| style = "vertical-align: top" | <span style="opacity:.5">octave:4> </span>A = infsup (A, …); <span style="opacity:.5">octave:5> </span>A \ [3; 0] # no solution ans = 2×1 work as expected on interval vector [Empty] [Empty] data types, the function <span style="opacity:.5">octave:6code> isempty</spancode>A \ [4; 8] # many solutions ans = 2×1 is evaluated element-wise and checks if an interval vector [4] [Entire]|} === Error handling ===Due to equals the nature of empty set-based interval arithmetic, one should never observe errors (in the sense of raised GNU Octave error messages) during computation. If you do, there either is a bug in the code or there are unsupported data types. Arithmetic operations which are not defined for (parts of) their input, simply ignore anything that is outside of their domain.  <span style="opacity:.5">octave:1> </span>infsup builtin (2, 3) / 0 ans = [Empty] <span style="opacity:.5isempty">octave:2> </span>infsup , empty (0) ^ infsup (0) ans = [Empty] However, the interval constructors can produce errors depending on the input. The <code>infsup</code> constructor will fail if the interval boundaries are invalid. Contrariwise, the <code>infsupdec</code> constructor will only issue a warning and return a [NaI], which will propagate and survive through computations.  <span style="opacity:.5">octave:3> </span>infsup isempty (empty (3, 2) + 1 error: illegal interval boundaries: infimum greater than supremum ''… (call stack) …'' <span styleans ="opacity:.5">octave:3> </span>infsupdec (3, 2) + 1 warning: illegal interval boundaries: infimum greater than supremum0 ans = [NaI] 1
== IEEE 1788 index ==
240

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