def f(x): evaluvates individually but not inside plot

asked 2012-11-28 13:11:11 +0100

indiajoe
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updated 2023-01-09 23:59:31 +0100

tmonteil
27423 ●31 ●204 ●517 http://wiki.sagemath.o...

Hi, I am stuck with a curious error. The code to reproduce error is given below. The function T basically integrates the function with a given value of L.

def T(L): return n(integral(4/sqrt(pi)*exp(-L)*(x^2)*exp(-x^2/4)/cosh(sqrt(L)*x),x,0,+Infinity))
plot(T,5,7)

The function T works fine if I ask

print T(5)

But it gives error, when i give the above mentioned plot command. What might be going wrong here?

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answered 2012-11-28 15:11:05 +0100

calc314
4191 ●22 ●48 ●113

Using numerical_integral will allow you to produce a plot as follows.

def T(L): 
    ans=numerical_integral(4/sqrt(pi)*exp(-L)*(x^2)*exp(-x^2/4)/cosh(sqrt(L)*x),0,oo)
    return ans[0]

plot(lambda L: T(L),(L,5,7))

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@calc314 Thankyou very much. This works for me. Though I am still curious why the integral() function didn't work.

indiajoe ( 2012-11-28 17:04:33 +0100 )edit

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answered 2012-11-28 13:44:04 +0100

calc314
4191 ●22 ●48 ●113

I can't get T to evaluate for noninteger points. For example, T(5.3) gives the following error.

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "_sage_input_22.py", line 10, in <module>
    exec compile(u'open("___code___.py","w").write("# -*- coding: utf-8 -*-\\n" + _support_.preparse_worksheet_cell(base64.b64decode("VCg1LjMp"),globals())+"\\n"); execfile(os.path.abspath("___code___.py"))
  File "", line 1, in <module>

  File "/tmp/tmpudPhvP/___code___.py", line 3, in <module>
    exec compile(u'T(_sage_const_5p3 )
  File "", line 1, in <module>

  File "/tmp/tmp4oZQrZ/___code___.py", line 5, in T
    return integral(_sage_const_4 /sqrt(pi)*exp(-L)*(x**_sage_const_2 )*exp(-x**_sage_const_2 /_sage_const_4 )/cosh(sqrt(L)*x),x,_sage_const_0 ,oo)
  File "/home/sageserver/sage-5.0.1/local/lib/python2.7/site-packages/sage/misc/functional.py", line 728, in integral
    return x.integral(*args, **kwds)
  File "expression.pyx", line 8745, in sage.symbolic.expression.Expression.integral (sage/symbolic/expression.cpp:33707)
  File "/home/sageserver/sage-5.0.1/local/lib/python2.7/site-packages/sage/symbolic/integration/integral.py", line 633, in integrate
    return definite_integral(expression, v, a, b)
  File "function.pyx", line 413, in sage.symbolic.function.Function.__call__ (sage/symbolic/function.cpp:4678)
  File "/home/sageserver/sage-5.0.1/local/lib/python2.7/site-packages/sage/symbolic/integration/integral.py", line 173, in _eval_
    return integrator(*args)
  File "/home/sageserver/sage-5.0.1/local/lib/python2.7/site-packages/sage/symbolic/integration/external.py", line 21, in maxima_integrator
    result = maxima.sr_integral(expression, v, a, b)
  File "/home/sageserver/sage-5.0.1/local/lib/python2.7/site-packages/sage/interfaces/maxima_lib.py", line 747, in sr_integral
    raise error
RuntimeError: ECL says: In function GCD, the value of the second argument is
  1.0
which is not of the expected type INTEGER

From some experimenting, the problem appears to be in cosh. If I take the sqrt(L) out of cosh, then it will evaluate. This seems odd.

In the meantime, you might using a combinations of exponentials in place of cosh to force the computation to work.

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Using exponentials also does not work when sqrt(L) is not an integer.

calc314 ( 2012-11-28 13:48:42 +0100 )edit

Huh. Note that `sage: n(integral(4/sqrt(pi)*exp(-5.3)*(x^2)*exp(-x^2/4)/cosh(sqrt(53/10)*x),x,0,oo))` gives 0.00231492212520274.

kcrisman ( 2012-11-28 14:25:48 +0100 )edit

But I'm having trouble getting a Maxima command that will reproduce this error with `5.3`.

kcrisman ( 2012-11-28 14:27:50 +0100 )edit

The following doesn't work either: `integrate(exp(-5.3*x),x,0,1)` Somehow the float isn't being passed somewhere correctly. Why does the error message mention the GCD function? I can't figure out why that function is involved here.

calc314 ( 2012-11-28 19:05:33 +0100 )edit

The error is coming from ECL, so that indicates the problem may be somewhere in maxima.

Jason Grout ( 2012-11-28 21:48:43 +0100 )edit

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