Van der Corput's constant is given by
(OEIS A143305), where and are Fresnel integrals,
and is the unique negative root of
with , namely
(Finch 2003, p. 245; OEIS A143306).
More things to try:
Weisstein, Eric W. "van der Corput's Constant." From MathWorld--A Wolfram Resource. https://mathworld.wolfram.com/vanderCorputsConstant.html