Using fsolve to solve a system of multi-variable equations

I am trying to solve a system of 4 equations which are summations of Eigenseries. There are 4 variables. The functions/governing equations are actually determine using several inputs which are pre-determined constants using a function. It seems that when I'm trying to use the function handle (@ funk) I am not allowed to put any inputs other than 'x', a vector that contains 4 the variables, into funk. How am I supposed to go around this? I need the equations to be inputs into the function or to somehow use fsolve on 4 equations direction without using a function.

Thank you for your time, Alex.

Answer 1

[Torsten]

https://de.mathworks.com/help/optim/ug/passing-extra-parameters.html

Best wishes

Torsten.

[Alex Wylie]

Wow you are very fast, thanks a lot. I just tried to use global variables however I got the error

Error using fsolve (line 269)
FSOLVE requires all values returned by functions to be of data type double.

and I can't convert the globals to doubles in the function. Any idea around this if this is the technique you were referring to ? If it is not I will try the other techniques, that is nested functions and anonymous functions, now. Thanks a lot Torsten

[Torsten]

Could you please show us the relevant part of your code ?

Best wishes

Torsten.

[Alex Wylie]

I want to solve these 4 equations for the 4 variables which as syms: a_i, b_i, c_i and d_i. The functions just calculate a coefficient for the respective variables. I have tried vpasolve and solve and it doesn't work ): I am trying to work out how to do it with nested functions but am a little stuck.

EQ_1 = dphi1(a_inc, a_i, Cn, kf, nf, -B/2, B, w, t, n) - dphi2(b_i, c_i, Cn, kr, nr, -B/2, B, w, t, n);
EQ_2 = phi1(a_inc, a_i, Cn, kf, nf, -B/2, B, w, t, n) - phi2(b_i, c_i, Cn, kr, nr, -B/2, B, w, t, n);
EQ_3 = dphi2(b_i, c_i, Cn, kr, nr, B/2, B, w, t, n) - dphi3(d_i, Cn, kf, nf, B/2, B, w, t, n);
EQ_4 = phi2(b_i, c_i, Cn, kr, nr, B/2, B, w, t, n) - phi3(d_i, Cn, kf, nf, B/2, B, w, t, n);

Thanks for your time

[Alex Wylie]

I think I may have fixed it but I have a different error now. I used the code

x = [a_i, b_i, c_i, d_i];
fun = @(x) funk(a_inc, x, Cn, kf, kr, nf, nr, B, w, n, t);
x0 = double([a_inc*0.3, a_inc*0.7, a_inc*0.1, a_inc*0.6]);
x = fsolve(fun, x0);

Where the function funk is

function fun = funk(a_inc, x, Cn, kf, kr, nf, nr, B, w, n, t)

EQ_1 = dphi1(a_inc, x(1), Cn, kf, nf, -B/2, B, w, t, n) - dphi2(x(2), x(3), Cn, kr, nr, -B/2, B, w, t, n);
EQ_2 = phi1(a_inc, x(1), Cn, kf, nf, -B/2, B, w, t, n) - phi2(x(2), x(3), Cn, kr, nr, -B/2, B, w, t, n);
EQ_3 = dphi2(x(2), x(3), Cn, kr, nr, B/2, B, w, t, n) - dphi3(x(4), Cn, kf, nf, B/2, B, w, t, n);
EQ_4 = phi2(x(2), x(3), Cn, kr, nr, B/2, B, w, t, n) - phi3(x(4), Cn, kf, nf, B/2, B, w, t, n);
fun = [EQ_1, EQ_2, EQ_3, EQ_4];
end

However now I get the error: "Error using fsolve (line 269) FSOLVE requires all values returned by functions to be of data type double." Once again I tried to convert each of the inputs excluding 'x' into double but the function double didn't let me. Any ideas ? Thanks so much

[Torsten]

fsolve doesn't work with symbolic variables - EQ_1, EQ_2, EQ_3 and EQ_4 must be of type double.

Best wishes

Torsten.

Answer 2

[Abdullaziz Al-Farqani]

try to define the 4 variables as vectors (x(1),x(2),...) instead of syms: a_i, b_i, c_i and d_i. See if it works