problem with psi digamma function

Can anyone help? I try to use the psi function to evaluate a complex number. It suggests in the help that I should convert number to sym first and then evaluate. I tried the examples in the math works page: http://www.mathworks.co.uk/help/toolbox/symbolic/psi.html

This works: [psi(1/2) psi(2, 1/2) psi(1.34) psi(1, sin(pi/3))] ans =

   -1.9635  -16.8288   -0.1248    2.0372

But then when I tried this:

[psi(sym(1/2)), psi(1, sym(1/2)), psi(sym(1/4))] ??? Error using ==> psi Input must be single or double.

Please any help with this?

Answer 1

[Walter Roberson]

Sorry, system ate my earlier response:

Your expression psi(1, sym(1/2)) has a double precision number as the first parameter, so the MATLAB psi routine is the one that is dispatched. And then that routine complains because the second parameter is not a floating point number.

Cure: psi(sym(1), sym(1/2))

Better yet: psi(sym(1), sym(1)/sym(2))

or psi(sym(1), sym('1/2'))

Remember, sym() with a numeric expression as a parameter will evaluate the numeric expression before passing it to sym. sym() on a quoted will treat the expression in the string symbolically.

[Ali]

Thanks!

I tried your suggestion but got this:

psi(sym(1), sym(1/2))

??? Error using ==> psi

Too many input arguments.

I suspect I have a version of matlab that doesn't have the updated psi function? I am using 2010a and saw since I posted this question that the psi function was updated in the 2011 version.

[Walter Roberson]

Looks like there was no direct interface to psi in R2010a; it is not in the Special Functions list http://www.mathworks.com/help/releases/R2010a/toolbox/symbolic/f3-157665.html#f3-15507

psi does exist in MuPad in R2010a and can be invoked through

feval(symengine, 'psi', sym(1/2), sym(1))

http://www.mathworks.com/help/releases/R2010a/toolbox/mupad/stdlib/psi.html

Please note that this used the arguments in the other order than the R2011 MATLAB psi() interface to the MuPAD psi() routine.

Answer 2

[Kai Gehrs]

Hi Ali,

just for the future: you may want to have a look at

 mfunlist

in the Symbolic Math Toolbox interface in MATLAB. This gives access to special functions available in MuPAD by a more convenient syntax than maybe the

   evalin

command does. For example

 mfun('Psi',2,4)

corresponds to

psi(2,4)

but internally calls MuPAD's implementation of the PSI function.

Hope this helps a bit and best regards,

-- Kai