MATLAB Answers

Igor
0

Symbolic Toolbox: integration

Asked by Igor
on 12 Dec 2011
>> int('cos(x^2)^2')
Warning: Explicit integral could not be found.
ans =
int(cos(x^2)^2, x)
>> int('cos(2*x^2)')
ans =
(pi^(1/2)*fresnelC((2*x)/pi^(1/2)))/2
But there is a formula cos(2y)=2cos(y)^2)-1 and in first case MATLAB can't solve..
May be MuPAD forced to solve?

  9 Comments

int(simple(sym('cos(x^2)^2')),'x')
Andrei Bobrov
on 13 Dec 2011
I'm agree , Walter!
(Maple Toolbox)
http://imageshack.us/photo/my-images/843/symint3.png/
Andrei Bobrov
on 13 Dec 2011
Last comment :)
http://imageshack.us/photo/my-images/402/symint4.png/
(for Maple Toolbox)

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1 Answer

bym
Answer by bym
on 14 Dec 2011
 Accepted Answer

Apparently you have to do the substitution manually
int(simple(cos(x^2)^2))
Warning: Explicit integral could not be found.
Using the sincos version returned by
simple(cos(x^2)^2)
yields this
int(cos(2*x^2)/2 + 1/2)
ans =
x/2 + (pi^(1/2)*fresnelC((2*x)/pi^(1/2)))/4
I could not find where the sincos 'simplification' is available outside the simple command as it is in Maple...maybe this is the reason Maple returns the result automagically ;)
[edit for completeness]
int(feval(symengine, 'combine', cos(x^2)^2, 'sincos'))
ans =
x/2 + (pi^(1/2)*fresnelC((2*x)/pi^(1/2)))/4

  4 Comments

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bym
on 14 Dec 2011
works well, my local documentation does not turn up an obvious help topic when searching for 'combine'
feval(symengine, 'combine', cos(x^2)^2, 'sincos')
ans =
cos(2*x^2)/2 + 1/2
thanks Walter
Igor
on 14 Dec 2011
Yes, yes...
good construction with "combine"/"sincos"
But from the technical calculation point of view (as semi-analitical and fast integration is needed)
it's not good to think about integration procedure details...
And if more complex integral occures??
To ask own intuition?
Humans are better at seeing patterns or understanding context than machines are. It is *often* the case that a human will have to apply knowledge of theoretical equivalences in order to make progress in integration.
MuPAD is not as strong as some of the other products around, but _all_ of them are missing a lot of patterns.

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