使用fzero:类型为'sym'的输入参数的未定义函数或方法'isfinite' [英] Using fzero: Undefined function or method 'isfinite' for input arguments of type 'sym'

问题描述

我使用matlab进行符号计算.经过长时间的计算，我得到了一个x函数，它是贝塞尔函数的组合，我想找到它的零.

为此，我在Matlab中使用fzero函数.但是，尽管它非常适合单个贝塞尔函数，但不适用于我的一个.

为什么会这样?该如何解决?

预先感谢

解决方案

我认为您误以为符号表示形式的函数句柄.

fzero需要一个函数句柄.

因此，如果您这样做:

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)

你得到

3.8173

请注意，如果要调用符号函数，则必须间接执行:

fzero(@(x)eval(DELTA_xi), 3.5)

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

解决方案

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)

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