OK, I figured out how to get an answer that is reproducible in another
software (PyIMSL's zeroFunction routine, which uses a combination of
Brent and Muller's methods)!
In [100]: mpmath.findroot(lambda w: 1e80*exp(w) - 1e-300, (-1000,0),
solver='bisect')
Out[100]: -874.982335337737
I am still a poor sympy user though. For example, with
In [101]: mpmath.findroot(lambda w: 1e80*exp(w) - 1e-300, (-1000,0),
solver='anderson')
Out[101]:
-1000.0
1.0e-300 - 1.0e+80*e
-1000.0 + ---------------------------
-1000.0
1.0e+77 - 1.0e+77*e
How do I get a numerical evaluation of this expression?
And, when testing Ridder's method, which is what I want to compare
apples with apples in Scipy, R etc, I get the dreaded
TypeError: cannot create mpf from...
message, and I have no idea what this means, though I keep managing to
generate it as I play around.
In [102]: mpmath.findroot(lambda w: 1e80*exp(w) - 1e-300, (-1000,0),
solver='ridder')
---------------------------------------------------------------------------
TypeError Traceback (most recent call
last)
C:\Jabberwocky\examples\<ipython console> in <module>()
C:\Program Files\EPD_511\Lib\site-packages\sympy\mpmath\mptypes.pyc in
g(*args, **kwargs)
66 return +v
67 else:
---> 68 return f(*args, **kwargs)
69 finally:
70 self.mp.prec = orig
C:\Program Files\EPD_511\Lib\site-packages\sympy\mpmath
\optimization.pyc in findroot(f, x0, solver, tol, verbose, verify,
force_type, **kwargs)
942 maxsteps = iterations.maxsteps
943 i = 0
--> 944 for x, error in iterations:
945 if verbose:
946 print 'x: ', x
C:\Program Files\EPD_511\Lib\site-packages\sympy\mpmath
\optimization.pyc in __iter__(self)
483 x3 = 0.5*(x1 + x2)
484 fx3 = f(x3)
--> 485 x4 = x3 + (x3 - x1) * sign(fx1 - fx2) * fx3 /
sqrt(fx3**2 - fx1*fx2)
486 fx4 = f(x4)
487 if abs(fx4) < self.tol:
C:\Program Files\EPD_511\Lib\site-packages\sympy\mpmath\functions.pyc
in sign(ctx, x)
251
252 """
--> 253 x = ctx.convert(x)
254 if not x or ctx.isnan(x):
255 return x
C:\Program Files\EPD_511\Lib\site-packages\sympy\mpmath\mptypes.pyc in
convert(ctx, x, strings)
361 if hasattr(x, '_mpmath_'):
362 return ctx.convert(x._mpmath_(*prec_rounding))
--> 363 raise TypeError("cannot create mpf from " + repr(x))
364
365 def isnan(ctx, x):
TypeError: cannot create mpf from -1.0e+80 + 1.0e+80*exp(-1000.0)
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