Hello, Sage Community! Trying to answer this related question, I think I have found a bug along with user @Nasser. Although my answer there explains what I think to be wrong, let me summarize it here.
When calling
integrate(x+1,x, algorithm="giac")
Sage works perfectly, but when calling
integrate((1-2*x^(1/3))^(3/4)/x,x, algorithm="giac")
we get the following traceback:
---------------------------------------------------------------------------
SyntaxError Traceback (most recent call last)
/Scientific/SageMath/local/lib/python3.7/site-packages/sage/interfaces/giac.py in _sage_(self, locals)
1101 return symbolic_expression_from_string(result, lsymbols,
-> 1102 accept_sequence=True)
1103
/Scientific/SageMath/local/lib/python3.7/site-packages/sage/calculus/calculus.py in symbolic_expression_from_string(s, syms, accept_sequence)
2384 _augmented_syms = syms
-> 2385 return parse_func(s)
2386 finally:
/Scientific/SageMath/local/lib/python3.7/site-packages/sage/misc/parser.pyx in sage.misc.parser.Parser.parse_sequence (build/cythonized/sage/misc/parser.c:5479)()
537
--> 538 cpdef parse_sequence(self, s):
539 """
/Scientific/SageMath/local/lib/python3.7/site-packages/sage/misc/parser.pyx in sage.misc.parser.Parser.parse_sequence (build/cythonized/sage/misc/parser.c:5369)()
555 if tokens.next() != EOS:
--> 556 self.parse_error(tokens)
557 if len(all) == 1 and isinstance(all, list):
/Scientific/SageMath/local/lib/python3.7/site-packages/sage/misc/parser.pyx in sage.misc.parser.Parser.parse_error (build/cythonized/sage/misc/parser.c:9742)()
1006 cdef parse_error(self, Tokenizer tokens, msg="Malformed expression"):
-> 1007 raise SyntaxError(msg, tokens.s, tokens.pos)
1008
SyntaxError: Malformed expression
During handling of the above exception, another exception occurred:
NotImplementedError Traceback (most recent call last)
<ipython-input-92-987ddabbc645> in <module>()
----> 1 integrate((Integer(1)-Integer(2)*x**(Integer(1)/Integer(3)))**(Integer(3)/Integer(4))/x,x, algorithm="giac")
/Scientific/SageMath/local/lib/python3.7/site-packages/sage/misc/functional.py in integral(x, *args, **kwds)
751 """
752 if hasattr(x, 'integral'):
--> 753 return x.integral(*args, **kwds)
754 else:
755 from sage.symbolic.ring import SR
/Scientific/SageMath/local/lib/python3.7/site-packages/sage/symbolic/expression.pyx in sage.symbolic.expression.Expression.integral (build/cythonized/sage/symbolic/expression.cpp:64541)()
12370 R = ring.SR
12371 return R(integral(f, v, a, b, **kwds))
> 12372 return integral(self, *args, **kwds)
12373
12374 integrate = integral
/Scientific/SageMath/local/lib/python3.7/site-packages/sage/symbolic/integration/integral.py in integrate(expression, v, a, b, algorithm, hold)
917 if not integrator:
918 raise ValueError("Unknown algorithm: %s" % algorithm)
--> 919 return integrator(expression, v, a, b)
920 if a is None:
921 return indefinite_integral(expression, v, hold=hold)
/Scientific/SageMath/local/lib/python3.7/site-packages/sage/symbolic/integration/external.py in giac_integrator(expression, v, a, b)
430 return expression.integrate(v, a, b, hold=True)
431 else:
--> 432 return result._sage_()
/Scientific/SageMath/local/lib/python3.7/site-packages/sage/interfaces/giac.py in _sage_(self, locals)
1103
1104 except Exception:
-> 1105 raise NotImplementedError("Unable to parse Giac output: %s" % result)
1106 else:
1107 return [entry.sage() for entry in self]
NotImplementedError: Unable to parse Giac output: Evaluation time: 1.18
12*(1/4*ln(abs((-2*x^(1/3)+1)^(1/4)-1))-1/4*ln((-2*x^(1/3)+1)^(1/4)+1)+1/2*atan((-2*x^(1/3)+1)^(1/4))+1/3*((-2*x^(1/3)+1)^(1/4))^3)
Here is what I could deduce after reading a lot of source code and playing with examples: When we call something like this in Sage, the following process occurs at low level:
ex = (x+1)._giac_()
result = ex.integrate(x._giac_())
result._sage_()
The first line converts the Sage expression to Giac representation; the second calls the Giac integrate function, with respect to the variable x, which must also be converted to Giac representation (that's the x._giac_()); finally, the third line converts the result back to Sage representation (for example, replacing ln from Giac to log from Sage). This last step seems to be the problem. In this particular case, print(result) shows
x^2/2 + x
However, with a more complex expression, which takes more time to be integrated:
ex=((1-2*x^(1/3))^(3/4)/x)._giac_()
result = ex.integrate(x._giac_())
print(result)
the same process gives a different result:
Evaluation time: 1.19
12*(1/4*ln(abs((-2*x^(1/3)+1)^(1/4)-1))-1/4*ln((-2*x^(1/3)+1)^(1/4)+1)+1/2*atan((-2*x^(1/3)+1)^(1/4))+1/3*((-2*x^(1/3)+1)^(1/4))^3)
Notice the "Evaluation time" bit that was absent in the previous example? This is Giac saying it took more time than usual to perform the integration. When that string makes presence is when Sage fails:
result._sage_()
will raise the exception in the traceback.
Can somebody confirm this is a bug?
