Error: "fraction must have unit denominator" when converting symbolic expression into Univariate Polynomial Ring
 
  
 
  
 
 
 
 
 
 
    
        
        asked 2 years ago  
 
 IvanaGyro gravatar image  
 
 
 
 
    
        
        updated 1 year ago  
 
 FrédéricC gravatar image  
 
 
 
 
 
I want to convert a symbolic expression into (fractional) polynomials over RealField.
 
ploy_real = RealField()['E']
(E_real,) = ploy_real._first_ngens(1)
E = SR.var('E')
f = -1/4*(4*E + 3)/E
f.polynomial(base_ring=RealField())
 
However, I got this error. How to solve this error? Hasn't f already had a denominator E?
 
/usr/lib/python3/dist-packages/sage/rings/fraction_field.py in _call_(self, x, check)
   1242             # However, too much existing code is expecting this to throw a
   1243             # TypeError, so we decided to keep it for the time being.
-> 1244             raise TypeError("fraction must have unit denominator")
   1245         return num * den.inverse_of_unit()
 
Preview: (hide)
 
 
 
 
          
 
Comments
Your expression cannot be a polynomial. Consider :
sage: f=1/4*(4*x+3)/x ; f
1/4*(4*x + 3)/x
sage: f.expand()
3/4/x + 1
i. e. 1+34x, which is not a (finite) polynomial. You may try : 
sage: with assuming(x>0,x<1): f==f.expand().subs((sum(x^p, p, 0, oo)==sum(x^p, p, 0, oo, hold=True)).subs(x==1-x))
1/4*(4*x + 3)/x == 3/4*sum((-x + 1)^p, p, 0, +Infinity) + 1
i. e.
4x+34x=34+∞∑p=0(−x+1)p+1 
valid for x∈(0 1).
But this "polynomial" has an infinite number of terms...
Emmanuel Charpentier gravatar imageEmmanuel Charpentier (
    
        2 years ago
    )
I see. I found what I want is f.fraction(RealField()) Thank you.
IvanaGyro gravatar imageIvanaGyro (
    
        2 years ago
    )
I see. BTW, ypu can get that more easily, along the lines of :
sage: f
1/4*(4*x + 3)/x
sage: f.laurent_polynomial(SR)
3/4*x^-1 + 1
HTH,
Emmanuel Charpentier gravatar imageEmmanuel Charpentier (
    
        2 years ago
    )
add a comment