The most common method of generating the random number sequence
over 
is known as the
linear congruent or power residue method. Multiply the previous
random number 
by the constant a, add on another constant
c, take the modulus by M, and then keep just the fractional
part (remainder) as the next random number 
:
In (1), integer arithmetic is used to avoid round off error,
the value for 
(the seed) is often supplied by the user,
and mod(n,m) is often a built-in function on your computer for
remaindering (it may be called amod, dmod).
For example, if the subroutine has c=1, a=4, M=9 built in, and you
supply 
, then you obtain the sequence:
Note that you obtain the same result for the mod operation by just subtracting M until any further subtractions would leave a negative number (what remains is the remainder).
The rule (1) produces integers in
the range 
- but not necessarily every one. If a particular
integer comes up a second time, then the whole cycle repeats. In order
to obtain a long sequence and to have a rather random remainder, M
and a should be large numbers - but not so large that aM causes an
overflow.
Your computer probably has a built in random number generator which may well be better than the one you choose by chance. You should check this out in the manual pages (try the man command) and then test the generated sequence. These routines often have names like rand, rn, random, srand, erand, or drand. Here are two examples of how to call these routines in C and FORTRAN.
On the AIX system, the drand48 routine generates random numbers
in 
with good spectral properties. It uses
and 48 bit integer arithmetic. Make sure to initialize it properly by calling the subroutine srand48 to plant your seed. To be safe, you can reseed during your calculation (read manual for details).
The definition (1) will generate 
's in the range
. If random numbers in the range 
are needed, you need
only scale, for example:
Here too, floating point numbers should be used to avoid truncation.
