syms y(x) x Y
r=0.05;
m=0.01;
p=1;
s=1;
a=0.25;
b=0.25;
Dy = diff(y);
D2y = diff(y,2);
ode= y-1/x*1/(r-((p*Dy*x*(s^2))/y)+((D2y*x*(s^2))/(2*y))+((Dy*(m-(s^2)))/y))^(1/(1-a-b))
Notice that the denominator has the second derivative of y(x), and notice that the entire term is squared -- so the square of the second derivative of y(x) is used in the ODE.
The error is saying that whatever the highest degree of derivative is used, that derivative must be used linearly -- it can be multiplied by constants but it cannot be raised to any power (other than 0, which would remove the term, or 1, which would leave the term unchanged.)
