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# Definite integrals with mp-objects

Hello,

Is there a possibility to calculate definite integrals of sum of products of sines/cosines with cosh/sinh with mp-objects?

Kind regards,

Jeremy

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Hello Jeremy,

Yes, of course. Toolbox provides full spectrum of numerical integration routines - from fixed Gauss quadrature to adaptive quadgk.

Or did you mean something else?

For example, I would like to calculate the definite integral "int(sin(b*x)*cosh(c*x)" in the variable x over the interval [0,L], in which b,c and L are constant values (which I defined as mp-objects).

Define your integrand as a function, call quadgk with corresponding parameters.

Make sure the values 0, L and tolerance are also of mp-type. Toolbox will detect the mp-objects and will call its own multiprecision version of quadgk.

Let me know how it works.

Here is simple example for demonstration:

Just use your own integrand and interval boundariesCan I also use this to create a matrix of solutions, since I need to calculate these integrals with varying coefficients b and c?

Yes, of course, toolbox follows standard Matlab syntax and rules.

If you can do something with standard Matlab - most likely it can be done with toolbox too.

Hi again,

I couldn't find your last comment (did you delete it?).

Just few comments on the code.

The function should be able to accept the vector-parameters (quadqk is a vectorized method). So instead of

use element-wise multiplication:

Also be careful with precision and interval counts. My example shows extreme values, please use more reasonable at first, e.g.:

Quadgk will warn you if it needs more levels of recursion (MaxIntervalCount)

Thank you for your assistance! You've been of great help!

Hi again,

I can't seem to numerically stabilize my code (see below). I expect the diagonal elements in the matrix "Ortho" to be equal to L (here = 10) and the off-diagonal elements equal to zero. I'm trying to prove the orthogonality of the function f...

mp.Digits(20);

N = mp(5);

fun = @(x) cos(x)*cosh(x)-1;

B = mp(zeros(N,1));

B(1,1) = fzero(fun,[4,5]);

B(2,1) = fzero(fun,[7,8]);

for k=3:N

B(k,1) = (2*(k+1)-1)*pi/2;

end

for i=1:N

b(i,1) = B(i,1)/L;

end

ksi = mp(zeros(N,1));

for i=1:N

r = B(i,1);

ksi(i,1) = -(cos(r)-cosh(r))/(sin(r)-sinh(r));

end

Ortho = mp(zeros(N,N));

for i=1:N

p = b(i,1);

q = ksi(i,1);

for j=1:N

r = b(j,1);

s = ksi(j,1);

f = @(x)(cos(p*x)+cosh(p*x)+q*sin(p*x)+q*sinh(p*x)).*(cos(r*x)+cosh(r*x)+s*sin(r*x)+s*sinh(r*x));

[q,errbnd] = quadgk(f,mp(0),L,'RelTol',100*mp.eps(),'AbsTol',100*mp.eps(), 'MaxIntervalCount', 100000);

Ortho(j,i) = q;

end

end

I am not sure, probably you need to use better precision (from numerical point of view).

But why don't you compute the integral analytically?

Try using Maple or Mathematica. I have just tried Maple - it gave long but sensible expression for the any r,p,q,s.