fzero function not working with symsum

clear all
clc
close all

y = 0.5;
u = 0.25;
syms n t
fun = @(t) (-u + y-2/pi*symsum(1/n*sin(n*pi*(1-y))*exp(-(n*pi)^2*t),n,1,inf));
fzero(fun,0)

I want to find the root of the following (variable: t). I get the following error message:

Error using num2str (line 47)

Input to num2str must be numeric.

Error in fzero>errHandler (line 581)

sprintf('%g',y),num2str(fy)));

Error in fzero (line 346)

[b,fval,exitflag,output] = errHandler(a,fa,intervaliter,iter,fcount,trace,buildOutputStruct);

Error in a4_1 (line 9)

fzero(fun,0)

What can I do different?

Answer 1

[Walter Roberson]

Your symsum() is returning a symbolic value, so your fun() is returning a symbolic value -- but fzero requires the function to return single or double.

You should double() the results of the symsum.

[Iver Brekken]

clear all
clc
close all

y = 0.5;
u = 0.25;
syms n x
s = double(symsum(1/n*sin(n*pi*(1-y))*exp(-(n*pi)^2*x),n,1,inf));
fun = -u + y-2/pi*s;
fzero(fun,0)

Now i get this error message:

Error using symengine

Unable to convert expression containing symbolic variables into double array. Apply 'subs' function first to substitute values for variables.

Error in sym/double (line 702)

Xstr = mupadmex('symobj::double', S.s, 0);

Error in a4_1 (line 8)

s = double(symsum(1/n*sin(n*pi*(1-y))*exp(-(n*pi)^2*x),n,1,inf));

[Walter Roberson]

format long g
y = 0.5;
u = 0.25;
syms n x
s = symsum(1/n*sin(n*pi*(1-y))*exp(-(n*pi)^2*x),n,1,inf)

s = 

fun = -u + y-2/pi*s

fun = 

Fun = matlabFunction(fun, 'vars', x)

Fun = function_handle with value:
    @(x)feval(@(n)symsum((exp(-n.^2.*x.*pi.^2).*sin((n.*pi)./2.0))./n,n,1.0,Inf),sym('n')).*(-6.366197723675814e-1)+1.0./4.0

FunW = @(x) double(Fun(x))

FunW = function_handle with value:
    @(x)double(Fun(x))

location = fzero(FunW, [.09 .1])

location = 
        0.0946869595678489