Don't write a whole mess of code, withput actually thinking about whether virtially the very first (significant) line you write produces a viable result.
syms x1 x2 x3 x4 t
tspan=[0 100];
[solx1 solx2 solx3 solx4] = solve([-x1+x2==0,-x1+x2.*x3+(0.0001+0.0001*x4.^4).*x2==0,1-x2.^2==0,x2==0], [x1, x2, x3, x4])
So now look carefully at the equations you have written. I'll spread them out into 4 lines:
-x1+x2==0 (1)
-x1+x2.*x3+(0.0001+0.0001*x4.^4).*x2==0 (2)
1-x2.^2==0 (3)
x2==0 (4)
Consider the last equation. That tells you that x2 == 0.
OOPS. Look at the third equation. From that if x2==0, is there a solution? That can only happen when 1==0, and from what I recall of mathematics, that is a rare event.
Seriusly, NO! Not ever. Never. Nada. Nil. Not gonna happen.
No solution exists for this system. Now, look back at what solve told you. Empty sym. I would look far more carefully at the problem you are trying to solve. Revisit how you generated these equations.
