I will note two other "issues" with the code. They might be artifacts of
having a cut-down sample or code still in development, but still I think
them worth pointing out.

First, and simplest, DX is never defined here. More subtle is that what
you have for X is an automatic local array. I'm not at all sure whether
that's what you want; there isn't enough context for me to tell, but I'm
suspicious that it isn't. An automatic local array always "goes away"
when the subroutine returns. That kind of behavior doesn't sound
consistent with the description of generating an "initial" array.

I would also like to reiterate and emphasize Beliavsky's suggestion to
use IMPLICIT NONE. Then declare the types of all variables (as you will
have to with that feature). I recommend never using the DIMENSION
statement at all; declare your X array with a REAL statement instead of
a DIMENSION statement. IMPLICIT NONE helps even experts in the language.
For newcomers such as your self, the help provided by IMPLICIT NONE is
even more important.

As a purely stylistic comment, this looks very reminiscent of 1970's
code, with the all caps, the lack of indentation, and the use of
implicit type declaration. The caps and indentation are of no great
consequence other than appearance (though there are times when
appearance matters and you wouldn't want people to think that the code
was from 30 years ago), but the implicit typing is a real issue.