public String toPString()
        {
                String str = new String();
                for(int n=0; n < a.size() ; n++)
                {
                        final BigInteger num = a.elementAt(n) ;
                        if ( num.compareTo(BigInteger.ZERO) != 0 )
                        {
                                str += " " ;
                                if ( num.compareTo(BigInteger.ZERO) > 0 && n> 0)
                                        str += "+" ;
                                str += a.elementAt(n).toString() ;
                                if ( n > 0 )
                                {
                                        str += "*x" ;
                                        if ( n > 1 ) 
                                                str += "^"+n ;
                                }
                        }
                }
                if ( str.length() == 0 )
                        str = "0" ;
                return str ;
        } /* toPString */