On the higher moments of the error term in the divisor problem

Let $\Delta(x)$ denote the error term in the Dirichlet divisor problem. Our main results are the asymptotic formulas $$\int_1^X \Delta^3(x){\rm d}x = BX^{7/4} + O_\epsilon(X^{\beta+\epsilon}) \qquad(B > 0)$$ and $$\int_1^X \Delta^4(x){\rm d}x = CX^2 + O_\epsilon(X^{\gamma+\epsilon}) \qquad(C > 0)$$ with $\beta = 7/5, \gamma = 23/12$. This improves on the values $\beta = 47/28, \gamma = 45/23$, due to K.-M. Tsang. A result on the integrals of $\Delta^3(x)$ and $\Delta^4(x)$ in short intervals is also proved.