Computable real numbers.

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Michael Stoll


Robert Smith <>


BSD 3-clause

Computable Reals


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.

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

(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 displayed 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 internal 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).

N.B. There are no external comparison functions, since comparison is in general not decidable in finite time. Instead, we recommend using ordinary Common Lisp comparison functions after producing a k-bit approximation using the RATIONAL-APPROX-R to convert a CREAL to a rational number.

Exported Functions and Constants

The following functions, constants and variables are exported. (The package is named "COMPUTABLE-REALS" or "CR" for short.)

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
RATIONAL-APPROX-R x:creal k:int>0
                       function    returns a rational approximation that
                                    differs by less than 2^(-k).
RATIONALIZE-R x:creal k:int>0
                       function    returns the simplest rational approximation
                                    that differs by less than 2^(-k)
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
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

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