# computable-reals /

Filename Size Date modified Message
734 B
4.3 KB
515 B
391 B
601 B
509 B
20.2 KB
493 B
```                       I N T R O D U C T I O N
=======================

Computable real numbers x are interpreted as (potentially) infinite
fractions in base 2 that are specified through a rule for computation
of an integer a with |(2^k)*x - a| <= 1 for any k>=0.

The internal data structure should not be accessed.
The interface for the outside world is as follows:

The type CREAL is a supertype of the type RATIONAL. (CREAL-P x), for
an object x, returns T if x is of type CREAL, otherwise NIL.

(APPROX-R x k), for a CREAL x and an integer k>=0, returns an integer
a with |(2^k)*x - a| < 1.

(MAKE-REAL fun) returns the real number given by fun. Here fun is a
function taking an argument k, that computes a as above.

CREALs are output by print etc. as a decimal fraction. The error hereby
is at most one unit in the last digit that was output. The number of
decimal digits after the decimal point is defined through the dynamic
variable *PRINT-PREC*.

For comparison operations etc. a precision threshold is used. It is
defined through the dynamic variable *CREAL-TOLERANCE*. Its value should
be a nonnegative integer n, meaning that numbers are considered equal
if they differ by at most 2^(-n).

E X P O R T E D   F U N C T I O N S   A N D   C O N S T A N T S
===============================================================

The following functions, constants and variables are exported. (The
package is named "REALS".)
CREAL                  type        type of the computable real numbers
CREAL-P object         function    tests for type CREAL
*PRINT-PREC*           variable    specifies precision of output
*CREAL-TOLERANCE*      variable    precision threshold for comparison
APPROX-R x:creal k:int>=0
function    returns approximation of x to k digits
MAKE-REAL function     function    creates object of type CREAL
RAW-APPROX-R x:creal   function    returns 3 values a,n,s with:
if a = 0: |x| <= 2^(-n), s = 0
and n >= *CREAL-TOLERANCE*
else: a0 integer > 4, n0 integer >=0,
s = +1 or -1, and sign(x) = s,
(a-1)*2^(-n) <= |x| <= (a+1)*2^(-n)
PRINT-R x:creal k:int>=0 &optional (flag t)
function    outputs x with k decimal digits.
If flag is true, first a newline.
+R {creal}*            function    computes the sum of the arguments
-R creal {creal}*      function    computes negative or difference
*R {creal}*            function    computes the product of the arguments
/R creal {creal}*      function    computes reciprocal or quotient
SQRT-R creal           function    computes the square root
LOG2-R                 constant    log(2)
PI-R                   constant    pi
2PI-R                  constant    2*pi
PI/2-R                 constant    pi/2
PI/4-R                 constant    pi/4
LOG-R x:creal &optional b:creal
function    computes the logarithm of n in base b;
default is the natural logarithm
EXP-R creal            function    computes the exponential function
EXPT-R x:creal y:creal function    computes x^y
SIN-R creal            function    computes the sine
COS-R creal            function    computes the cosine
TAN-R creal            function    computes the tangent
ATAN-R x:creal &optional y:creal
function    computes the arctangent of x or
the phase angle of (x,y)
ASH-R x:creal n:int    function    computes x * 2^n
ROUND-R x:creal &optional y:creal
function    computes two values q (integer) and r
(creal) with x = q*y + r and |r|<=|y|/2
according to the precision specified by
*CREAL-TOLERANCE*
FLOOR-R x:creal &optional y:creal
function    like ROUND-R, corresponding to floor
CEILING-R x:creal &optional y:creal
function    like ROUND-R, corresponding to ceiling
TRUNCATE-R x:creal &optional y:creal
function    like ROUND-R, corresponding to truncate

In the code, exported symbols at their location of definition are
emphasized in UPPERCASE.

```