Is it possible to obtain explicit symbolic solutions to such linear ordinary differential equations?

Question

The ordinary differential equations to solve have symbolic parameters $k_1,k_2,k_3,k_4,k_5,k_6 \in \mathbb{R}$. $$ \left\{ \begin{array}{l} {y_1}'(t)=-{k_1} {y_1}(t)-{k_2} {y_1}(t),\\ {y_2}'(t)={k_2} {y_1}(t)-{k_3} {y_2}(t),\\ {y_3}'(t)={k_1} {y_1}(t)+{k_3} {y_2}(t)-{k_4} {y_3}(t),\\ {y_4}'(t)={k_4} {y_3}(t)-{k_5} {y_2}(t) {y_4}(t)+{k_6} {y_5}(t),\\ {y_5}'(t)={k_5} {y_2}(t) {y_4}(t)-{k_6} {y_5}(t)\\ \end{array} \right.$$

The ODE system itself seems simple when all $k_i$ are numerical numbers. How to solve it symbolically ? Is it possible to obtain explicit solutions?

Update:

Expressions in MMA:(DSolve does not work for such nonlinear ODE system)

  DSolve[{D[y1[t], t] == -k1*y1[t] - k2*y1[t],
  D[y2[t], t] == k2*y1[t] - k3*y2[t],
  D[y3[t], t] == k1*y1[t] + k3*y2[t] - k4*y3[t],
  D[y4[t], t] == k4*y3[t] - k5*y2[t]*y4[t] + k6*y5[t],
  D[y5[t], t] == k5*y2[t]*y4[t] - k6*y5[t]}, {y1[t], y2[t], y3[t], 
  y4[t], y5[t]}, t]

Answer 1

This ODE system can't be solved symbolically with the given information.
First let's define the differential equations:

dg1 = y1'[t] == -k1 y1[t] - k2 y1[t]
dg2 = y2'[t] == k2 y1[t] - k3 y2[t]
dg3 = y3'[t] == k1 y1[t] + k3 y2[t] - k4 y3[t]
dg4 = y4'[t] == k4 y3[t] - k5 y2[t] y4[t] + k6 y5[t]
dg5 = y5'[t] == k5 y2[t] y4[t] - k6 y5[t]

Now the first three differential equations can be solved, because these are independent of the last two.

pSol = DSolve[{dg1, dg2, dg3}, {y1[t], y2[t], y3[t]}, t]

{{y1[t] -> E^((-k1 - k2) t) C[1], 
  y2[t] -> (E^(-k3 t) (-1 + E^((-k1 - k2) t + k3 t)) k2 C[1])/(-k1 - k2 + k3) + E^(-k3 t) C[2], 
  y3[t] -> (E^(-k3 t - k4 t) (E^(k3 t) k1^2 k3 - 
     E^((-k1 - k2) t + k3 t + k4 t) k1^2 k3 + 
     2 E^(k3 t) k1 k2 k3 - E^(k4 t) k1 k2 k3 - 
     E^((-k1 - k2) t + k3 t + k4 t) k1 k2 k3 + E^(k3 t) k2^2 k3 - 
     E^(k4 t) k2^2 k3 - E^(k3 t) k1 k3^2 + 
     E^((-k1 - k2) t + k3 t + k4 t) k1 k3^2 - E^(k3 t) k2 k3^2 + 
     E^((-k1 - k2) t + k3 t + k4 t) k2 k3^2 - E^(k3 t) k1^2 k4 + 
     E^((-k1 - k2) t + k3 t + k4 t) k1^2 k4 - E^(k3 t) k1 k2 k4 + 
     E^((-k1 - k2) t + k3 t + k4 t) k1 k2 k4 + E^(k3 t) k1 k3 k4 -
      E^((-k1 - k2) t + k3 t + k4 t) k1 k3 k4 + 
     E^(k4 t) k2 k3 k4 - 
     E^((-k1 - k2) t + k3 t + k4 t) k2 k3 k4) C[1])/((k1 + k2 - k3) (k1 + k2 - k4) (k3 - k4)) - 
     (E^(-k3 t - k4 t) (-E^(k3 t) + E^(k4 t)) k3 C[2])/(k3 - k4) + 
     E^(-k4 t) C[3]
}}

From the last differential equation we get

Solve[dg5, y5[t]]

{{y5[t] -> (k5 y2[t] y4[t] - y5'[t])/k6}}

This (or even easier to see k6 y5[t]) can be inserted into the fourth differential equation

dg4 /. (First@%)

y4'[t] == k4 y3[t] - y5'[t]

(Note that there is no y5[t] left.)
Inserting also pSol and applying Simplify provides the remaining equation

% /. (First@pSol) // Simplify

(E^(-(k3 + k4) t) k4 (E^((-k1 - k2 + k3 + k4) t) (k1 + k2) (k1 - k3) (k3 - k4) C[1] - 
  E^(k4 t) k3 (-k1 - k2 + k4) ((k1 - k3) C[2] + k2 (C[1] + C[2])) - 
  E^(k3 t) (k1 + k2 - k3) (k4 (k4 C[3] - k3 (C[2] + C[3])) + 
  k2 (-k4 C[3] + k3 (C[1] + C[2] + C[3])) + 
  k1 (-k4 (C[1] + C[3]) + k3 (C[1] + C[2] + C[3])))))/((k1 + k2 - k3) (k1 + k2 - k4) (k3 - k4)) + 
  y4'[t] + y5'[t] == 0

If one tries to solve this equation using

DSolve[%, {y4[t], y5[t]}, {t}]

the following message is received

DSolve::underdet: There are more dependent variables than equations, so the system is underdetermined. >>

The remaining equation can be solved for y4[t], though.

DSolve[%%, {y4[t]}, {t}]

{{y4[t] -> 
   C[4] + (E^(-(k1 + k2) t) (k1 - k3) (k3 - k4) k4 C[1] + 
   E^(-(k3 + k4) t) (-E^(k4 t) k4 (-k1 - k2 + k4) ((k1 - k3) C[2] + k2 (C[1] + C[2])) - 
   E^(k3 t) (k1 + k2 - k3) (k4 (k4 C[3] - k3 (C[2] + C[3])) + 
   k2 (-k4 C[3] + k3 (C[1] + C[2] + C[3])) + 
   k1 (-k4 (C[1] + C[3]) + 
   k3 (C[1] + C[2] + C[3])))) - (k1 + k2 - k3) (k1 + k2 - k4) (k3 - k4) y5[t])/((k1 + k2 - k3) (k1 + k2 - k4) (k3 - k4))
 }}

The ODE system can also be solved, e.g. if y5'[t] is a constant.

---

Additional note:
With the solutions of the first three equations (pSol), the remaining equations can be rewritten as

y4'[t] == k6 y5[t] - a[t] y4[t] + b[t]
y5'[t] == a[t] y4[t] - k6 y5[t]

Therefore the remaining problem is to solve this non-homogeneous first-order linear differential equation with variable coefficients.