Function 'subs' different output for a (similar?) input

Question

Bobby Fischer 2021-1-18

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此问题的直接链接

https://ww2.mathworks.cn/matlabcentral/answers/720395-function-subs-different-output-for-a-similar-input

回答： Divija Aleti 2021-1-25

clc
syms F(t)
F(t)=[-5*cos(2*t) -5*sin(2*t) 3*exp(2*t);
    -2*(cos(2*t)+sin(2*t)) 2*(cos(2*t)-sin(2*t)) 0;
    cos(2*t) sin(2*t) exp(2*t)]
invF=inv(F);
t=0;
invF0=subs(invF(t)) % Here
syms t
expAt=F(t)*invF0
DexpAt=diff(expAt)
t=0;
DexpA0=subs(DexpAt) % Here
% if I did this instead: DexpA0=subs(DexpAt(t)) gives error
% but if I do it above doesn't give error. Why?

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Answer 1

Divija Aleti 2021-1-25

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Hi Bobby,

syms F(t)

The above command implies that 'F' is a symbolic function. Hence, the command

invF0=subs(invF(t))

does not give error.

However, 't' is defined as a symbolic variable, and so, 'DexpAt' is also a symbolic variable. As variables cannot take any input arguments, the below command gives an error.

DexpA0=subs(DexpAt(t))

For the above command to run without any error, you can define 'DexpAt' also as a symbolic function by changing

DexpAt=diff(expAt)

to

DexpAt(t)=diff(expAt)

You can check your workspace to confirm if any symbol is a symbolic function or a symbolic variable.

For additional information on symbolic variables and functions, refer to the following link:

Create symbolic variables and functions - MATLAB syms (mathworks.com)

Regards,

Divija