How do you fix a 'function_handle' error?

I am trying to solve an IVP by using an ODESolver function that calls two other defined functions. I keep getting a message saying, "Undefined function 'ODESolver1' for input arguments of type 'function_handle'. " I don't know how to fix this.

Below is the ODESolver function that I have created

function[X,Y] = ODESolver1(Integrator,RHS,A,B,YA,DX)
    NI = int64(((B-A)/DX)+1);%Number of Integration Steps
    X = zeros(NI,1);
    Y = zeros(NI,1);
    %Initial Condition
    X(1)=A;
    Y(1)=YA;
    for i = 1:NI-1
        [X(i+1),Y(i+1)]=Integrator(X(i),Y(i),DX,RHS);
    end
end

And now this is my script that keeps getting me function_handle errors

[X1,Y1] = ODESolver1(@Euler,@HK20,0,0.2,0.4,0.01)
[X2,Y2] = ODESolver1(@Euler,@HK30,0,0.2,0.4,0.01)
[X3,Y3] = ODESolver1(@Euler,@HK40,0,0.2,0.4,0.01)

figure (1)
plot(X1,Y1,X2,Y2,X3,Y3)
xlabel('Time')
ylabel('H Concentration')
legend('K20','K30','K40')

[madhan ravi]

Upload other function Euler

[Adam]

Is ODESolver1 in a location on your path?

The 'for input arguments of type 'function_handle'' part of those type of error messages is usually a red herring and it simply cannot find any function of that name, whatever the inputs.

If it found the function but the first argument should be something other than a function handle you should get a different error to that.

[John Morabito]

This is my Euler function:

function [X,Y]=Euler(X0,Y0,DX,RHS)
    F=RHS(X0,Y0);
    Y=Y0+DX*F;
    X=X0+DX;
end

And this is my concentration function:

function [F] HK20(t,L)
K = 20;
L = .4*exp(-K*t);
M = -K*L;
end

[Jan]

There is a missing "=" in the definition of HK20()

Answer 1

[Jan]

Where is thze function ODESolver1 stored? In an M-files called "ODESolver1.m"? Is this stored in a folder, which is contained in the path? The error message means, that this function is not found, so either there is a typo in the file name or the parent folder is not contained in Matlab's path.