@nm There are a few problems here.

1. The subs in the original version must be used only for display purposes.

2. Latex fails  for  _Cn()
Latex(_C4()/x+x);   ==> \frac{1}{x}+x
Latex(_C4()+x);      ==> \mathit{\_C4} {}+x

3. It seems that in order to put c[k] instead of _Ck in front of a product, 
sort is not always useful, so, it will be necessary to use subsindets. 
 

@nm I think that this works.

assign(seq(`print/_C`||k, k=0..10) = seq(c[k], k=0..10)):
S:=[seq(_C||k = _C||k(), k=0..10)]:

sol:=dsolve(t^2*diff(diff(y(t),t),t)-t*diff(y(t),t)+5*y(t) = 0,y(t)):
subs(S, sol);

@JAMET Like this:

qq:=expand(q(3,b)); # a = arbitrary (a=3 here)
Delta:=diff(qq,x,y)^2-diff(qq,x,x)*diff(qq,y,y);
solve(Delta>0); 

Be aware that an ellipse could be complex, so an extended analysis is needed.
Why don't you try something yourself in Maple and show the results?

@JAMET In the context of my answer,

qq:=q(6,-6):
Delta:=diff(qq,x,y)^2-diff(qq,x,x)*diff(qq,y,y);

==> Delta := -108144 ==> ellipse

@Carl Love I agree in general, but not in this case. 

@JAMET 
1. Use the equations of the lines passing thru each pair of points.
2. You must find the equation of the conic first. Then you may use the geometry package.

@emendes Just curious: do you really need all the computations done in advance? Or is it a CPU test?
It is not difficult to write a compilable version, but actually Maple in not needed for this problem, so why not use e.g. C?

@Katatonia 

SYS1:=u>=0, r^2 - 2*r*c -r +2*c = u^2, -r+c+1+u>0,  0<=c, c<=1 , 0<r, r<1, r<2*c:
SolveTools:-SemiAlgebraic([SYS1], [r,u], parameters=[c]);

(Here, of course, the auxiliary variable u can be ignored when reading the answer.)

@s265344 I see now that you want real solutions. My answer is for complex ones and for your first version.
For the second one use Sys:=(lhs-rhs)~(sys);  but the conclusion is the same.
For real solutions it's more difficult; don't use a1>0 because the algorithms used by Maple are different and the system is too large for them.

For real solutions the DirectSearch approach could be OK but we must be sure not to obtain just an approximation of the null solution.

@Kitonum Unfortunately we should admit that this is just a speculation, Maple does not manage such problems. It does not work if we write {Limit(u(x, t), x=infinity)=0, Limit(u(x, t), x=-infinity)=0} or  {limit(u(_X, t), _X=infinity)=0, limit(u(_X, t), _X=-infinity)=0}.

Also, it does not work for A := int(diff(sin(x)*u(x,t),x), x=-infinity..infinity, continuous);

@mmcdara Once we know the cause, a workaround is simple. In your code, instead of instance := sol(...), just use:

instance := convert(sol(parameters=data), `global`);

@Earl The use of complex numbers is not essential, it is just a convenient way to obtain orthogonal coordinates (and conformal maps).

@Kitonum I think that the OP obviously knew the answer, and just wanted an explanation.

The question is not very clear. What means that f is continuous over an interval J? Must f be continuous at each point of J, or the restriction f|J must be continuous? How e removable discontinuity is it treated?

Note. Suppose that discont returns the empty set. We cannot conclude that f is continuous on R, because probably discont ignores the branches which define f (even if usually they are chosen such that f be continuous on R when possible).

@Assinat 

A^+ is the transpose of the matrix A. 

column-red.mw