On the four-dimensional divisor problem of $(a,b,c,c)$ type
On the four-dimensional divisor problem of $(a,b,c,c)$ type
In this paper we study the four-dimensional divisor problem of $(a,b,c,c)$ type, where $1\leq a\leq b<c$ are fixed integers. Our theorems improve some classical results. As an application, we study the error term of the summatory function of the exponential totient function.