We improve the error term in the van der Corput transform for exponential
sums
\sum_{a \le n \le b} g(n) exp(2\pi i f(n)).
For many functions g and f, we can extract the next term in the asymptotic,
showing that previous results, such as those of Karatsuba and Korolev, are
sharp. Of particular note, the methods of this paper avoid the use of the
truncated Poisson formula, and thus can be applied to much longer intervals
[a,b] with far better results. We provide a detailed analysis of the error term
in the case g(x)=1 and f(x)=(x/3)^{3/2}.Comment: 72 pages, 10 figure