I use matlab for symbolic calculations. After long calculations I've got a function of x, which is the combination of bessel functions and I want to find it's zeros.
For that purpose I use fzero function in Matlab. But while it works perfectly for single bessel functions, it wont work for the mine one.
>> fzero(@(x)besselj(0,x), 3.5)
ans =
2.4048
>> fzero(@(x)DELTA_xi, 3.5) ??? Undefined function or method 'isfinite' for input arguments of type 'sym'.
Error in ==> fzero at 333 elseif ~isfinite(fx) || ~isreal(fx)
>> DELTA_xi besseli(1, (3*x)/10)*besselj(1, (3*x)/10)*besselk(1, x)*bessely(0, x) - besseli(1, (3*x)/10)*besselj(1, (3*x)/10)*besselk(0, x)*bessely(1, x) - besseli(1, (3*x)/10)*bessely(1, (3*x)/10)*besselj(0, x)*besselk(1, x) + besseli(1, (3*x)/10)*bessely(1, (3*x)/10)*besselj(1, x)*besselk(0, x) - besselj(1, (3*x)/10)*besselk(1, (3*x)/10)*besseli(0, x)*bessely(1, x) - besselj(1, (3*x)/10)*besselk(1, (3*x)/10)*besseli(1, x)*bessely(0, x) + besselk(1, (3*x)/10)*bessely(1, (3*x)/10)*besseli(0, x)*besselj(1, x) + besselk(1, (3*x)/10)*bessely(1, (3*x)/10)*besseli(1, x)*besselj(0, x)
Why this happens? How to solve the issue?
Thanks in advance
I think you've mistaken function handles with symbolic representation.
fzero needs a function handle.
so if you do:
DELTA_xi = @(x) besseli(1, (3*x)/10)*besselj(1, (3*x)/10)*besselk(1, x)*bessely(0, x) - besseli(1, (3*x)/10)*besselj(1, (3*x)/10)*besselk(0, x)*bessely(1, x) - besseli(1, (3*x)/10)*bessely(1, (3*x)/10)*besselj(0, x)*besselk(1, x) + besseli(1, (3*x)/10)*bessely(1, (3*x)/10)*besselj(1, x)*besselk(0, x) - besselj(1, (3*x)/10)*besselk(1, (3*x)/10)*besseli(0, x)*bessely(1, x) - besselj(1, (3*x)/10)*besselk(1, (3*x)/10)*besseli(1, x)*bessely(0, x) + besselk(1, (3*x)/10)*bessely(1, (3*x)/10)*besseli(0, x)*besselj(1, x) + besselk(1, (3*x)/10)*bessely(1, (3*x)/10)*besseli(1, x)*besselj(0, x)
fzero(DELTA_xi, 3.5)
you get
3.8173
note that if you want to call a symbolic function, you'll have to do it indirectly:
fzero(@(x)eval(DELTA_xi), 3.5)