Errors while trying to solve a simple differential equation

Why am I getting all these errors while solving this simple differential equation?

 syms y t a b
eqn = diff(y,t,2) == a^2*y;
Dy = diff(y,t);
cond = [y(0)==b, Dy(0)==1];
ySol(t) = dsolve(eqn,cond)

??? Error using ==> sym.sym>checkindex at 2590
Index must be a positive integer or logical.

Error in ==> sym.sym>privformatscalar at 2540
  checkindex(x);

Error in ==> sym.sym>privformat at 2524
  s = privformatscalar(x);

Error in ==> sym.sym>sym.subsref at 1364
              [inds{k},refs{k}] = privformat(inds{k});

Answer 1

[Walter Roberson]

You need

syms y(t)

You are currently just using y so MATLAB do not know that it is a function and thinks it is a constant. Constant differentiated gives 0

[naygarp]

I previously tried going with 'syms y(t)' but it shows that the format is not recognized.

syms y(t) a b
eqn = diff(y,t,2) == a^2*y;
Dy = diff(y,t);
cond = [y(0)==b, Dy(0)==1];
ySol(t) = dsolve(eqn,cond)

??? Error using ==> syms at 61
Not a valid variable name.

[Walter Roberson]

You must be using a older version of MATLAB. Try

syms a b t
y = sym('y(t)');
y0 = subs(y,t,0);
Dy = diff(y,t);
Dy0 = subs(Dy, t, 0);
D2y = diff(Dy,t);
eqn = D2y == a^2*y;
cond = [y0 == b, Dy0 == 1];
ySol = dsolve(eqn, cond(1), cond(2))