Published online by Cambridge University Press: 25 November 2011
We discuss the moment condition for the fractional
functional central limit theorem (FCLT) for partial
sums of xt =
Δ−dut,
where is
the fractional integration parameter and
ut is weakly
dependent. The classical condition is existence of
q ≥ 2 and
moments of the innovation sequence. When
d is close to
this
moment condition is very strong. Our main result is
to show that when
and under
some relatively weak conditions on
ut, the existence of
moments is in fact necessary for the FCLT for
fractionally integrated processes and that
moments are necessary for more general fractional
processes. Davidson and de Jong (2000,
Econometric Theory 16, 643–666)
presented a fractional FCLT where only
q > 2 finite moments are
assumed. As a corollary to our main theorem we show
that their moment condition is not sufficient and
hence that their result is incorrect.
We are grateful to Benedikt Pötscher, three anonymous referees, and James Davidson for comments and to the Social Sciences and Humanities Research Council of Canada (SSHRC grant 410-2009-0183) and the Center for Research in Econometric Analysis of Time Series, (CREATES, funded by the Danish National Research Foundation) for financial support.