MATLAB: Substuting variable in a matrix

variable sustituting

I am trying to substitute the variable s in my B matrix by a number. However it doesnt work.

That could happen if you had assigned a value to s before you did the subs(). Try

subs(B, sym('s'), 1/sqrt(sym(3)) )

You could also try

    syms a b x
    assume(x== a+b);
    F = a^2+b^2+2*a*b+a+b+1;
    simplify(F)

For the first point, try turning off pretty printing

evalin(symengine, 'PRETTYPRINT := FALSE');

I do not promise that it will work, though. If it doesn't, then

char(a)

should show you the linear form.

I don't know why the decimal form is given for the hilbert matrix. You could try, though,

evalin(symengine, 'linalg::hilbert(2)');