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Approximate equality

This functionality does not run in MATLAB.

x ~= y_approx(x,y)

`x ~= y` symbolizes approximate equality.

`x ~= y` is equivalent to the function call `_approx(x,
y)`.

The operator `~=` returns a symbolic expression
representing an approximate equality for numerical values `x` and `y`.
The calls `bool(x ~= y)` and `is(x ~= y)` check
whether `|float((x - y)/x)| < 10^(-DIGITS)` is
satisfied, provided `x ≠ 0` and `y
≠ 0`. Thus, `TRUE` is returned if `x` and `y` coincide
within the relative numerical precision set by `DIGITS`. For `x
= 0`, the criterion is `|float(y)| < 10^(-DIGITS)`.
For `y = 0`, the criterion is `|float(x)|
< 10^(-DIGITS)`. If either `x` or `y` contains
a symbolic object that cannot be converted to a real or complex floating
point number, the functions `bool` and `is` return the value `UNKNOWN`.

Approximate equalities have two operands: the left hand side
and the right hand side. One may use `lhs` and `rhs` to extract these operands.

In the following, note the difference between syntactical and
numerical equality. The numbers 1.5 and
coincide
numerically. However, 1.5 is
of domain type `DOM_FLOAT`, whereas
is
of domain type `DOM_RAT`.
Consequently, they are not regarded as equal in the following syntactical
test:

1.5 = 3/2; bool(%)

If floating-point numbers are involved, one should rather use
the operator `~=` instead of `=`.
The functions `bool` and `is` test whether the
floating-point approximations coincide up to the relative precision
given by `DIGITS`:

1.5 ~= 3/2; bool(1.5 ~= 3/2); is(1.5 ~= 3/2);

The following expressions coincide syntactically:

_equal(1/x, diff(ln(x),x)); bool(%)

The Boolean operator `not` converts
equalities and inequalities:

not a = b, not a <> b

The examples below demonstrate how `=` and `<>` deal
with non-mathematical objects and data structures:

if "text" = "t"."e"."x"."t" then "yes" else "no" end

bool(table(a = PI) <> table(a = sqrt(2)))

We demonstrate the difference between the syntactical test via `bool` and the semantical
test via `testeq`:

bool(1 = x/(x + y) + y/(x + y)), testeq(1 = x/(x + y) + y/(x + y))

Equations and inequalities are typical input objects for system
functions such as `solve`:

solve(x^2 - 2*x = -1, x)

solve(x^2 - 2*x <> -1, x)

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