Recursion 递归级数多址阵列

我试图找到下列数列从n=1到n=无穷的和：

A(n)=T/(n+1)*{a1*T^2*A(n-3)/((n-1)*n) + a2*T*A(n-2)/n + a3*A(n-1)}

与

使用mupad。可能吗

我尝试了以下代码：

A := proc(n)
begin
  if(n<=0) then return(0) end_if:
  if(n=1) then return(a_1*T/2) end_if:
  return((T/(n+1))*(a_1*T^2*A(n-3)/((n-1)*n)
                    +a_2*T*A(n-2)/n
                    +a_3*A(n-1)))
end_proc:

mupad返回一个错误：

Error: Can't evaluate to boolean [_leequal];
during evaluation of 'A'

有没有更简单的方法来解决这个问题

sum(A(n),n=1..2)

Error: Can't evaluate to boolean [_leequal];
during evaluation of 'A'