why the output is a list type in Mupad ?

I have a rather complicated function "aa", which is a function of "A", "B", "C" and "DD". And "A", "C", "DD" are essentially functions of "B" (other variables, P, m0, Ka, Kd are constants).

I want to substitute the expressions of "A", "C" and "DD" into "aa" so it is only a function of "B". I do as follows in Mupad Notebook:

       A := 3/2*P*m0*Ka/(B*sinh(B)*(B^2+Kd) + Ka*B^2*cosh(B))
       C := -3/2*P*m0/B^2
       DD := -Ka*B^2/(B^2+Kd)

       aa := 1/B^4/(B*sinh(B)-DD*cosh(B))*[(1/3*A*B^3*DD - 1/4*A*B*DD - 1/4*A*B^3 + 1/4*A*B)*sinh(B) + (1/4*A*B^2*DD - 1/3*A*B^4 - 1/4*A*B^2)*cosh(B) + (B^2+1)*2*DD*C -2*B*C]

Mupad substitutes the expression of "A", "C" and "DD" into "aa", so it outputs:

The results is actually correct. However, "aa" right now converts to a list type (I check the type using domtype), which causes error if I want to do other mathematical manipulations on "aa" such as series.

Anyone can tell me why ? And how to deal with this ?

Answer 1

[Li Zhang]

The bracket in the "aa" expression causes all the problem. It converts "aa" into a list type although "looks like" just an bracket expression...