Thanks~actually the problem is a little bit more complicated, here is the original expression:
E^(-k22 t) ((0.5 b2 bb1^2 (-1 + E^(-k22 t)))/k22 - ( 0.5 b2 c1^2 (-1 + E^(k22 t)))/k22 - ( 0.5 a1^2 b2 (-1 + E^((k22 - 2 k33) t)))/(k22 - 2 k33) -...
Expand it and we get:
-((0.5 b2 c1^2)/k22) + (0.5 b2 bb1^2 E^(-2 k22 t))/k22 - ( 0.5 b2 bb1^2 E^(-k22 t))/k22 + (0.5 b2 c1^2 E^(-k22 t))/k22 + ( 0.5 a1^2 b2 E^(-k22 t))/(k22 - 2 k33) - ...
the items of the sum are all of the form:
b E^[a t] t^n
where a,b can be very complicated,
I want to put those items with the same a and n together, i.e, to simplify the expression to make sure that
only appear once in the sum, put the coefficients together in the bracket. Is this possible?