FUNCTION: SfAVars - specify valid variables in alphabets

CALLING SEQUENCE:

SfAVars(vset)

SFA[SfAVars](vset)

PARAMETERS:

vset = a set of variables

SYNOPSIS:

• The SfAVars function specifies which variables are considered for the SfAExpand function.

• vset gives the set of valid variables for SFA, for instance:
  { x1, x2, x3 },
  { {x}, {y} }, all the xi's and yi's,
  { {x}, {y}, t }, idem + the variable t.

• With no argument, SfAVars returns the current set of valid variables.

• Note that each call to SfAVars destroy the remember table of SfAExpand, if vset is specified.

• Whenever there is a conflict between the function name SfAVars and another name used in the same session, use the long form SFA['SfAVars'].

EXAMPLES:

> with(SFA):
> SfAVars({ {x}, t });
 
                                    {t, {x}}
 
> SfAExpand( s[3](x1+x2-t) );
 
                  2                 2     3        2     2        3
            - t x2  - t x1 x2 - t x1  + x2  + x1 x2  + x1  x2 + x1
 
> SfAVars({ {y}, t }):
> SfAExpand( s[3](x1+x2-t) );
 
             2                                2                    3
   - 1/2 t x2  - 1/2 t x2 - t x1 x2 - 1/2 t x1  - 1/2 t x1 + 1/6 x2
 
              2                     2                 2            3
      + 1/2 x2  + 1/3 x2 + 1/2 x1 x2  + x1 x2 + 1/2 x1  x2 + 1/6 x1
 
              2
      + 1/2 x1  + 1/3 x1
 
> SfAVars();
 
                                    {t, {y}}