Since
∣n−2x2∣<n−2|n^{-2}x^2|<n^{-2}∣n−2x2∣<n−2 in [-1/4, 1/3]
and
∑n−2\sum n^{-2}∑n−2 converges
then
∑n−2x2\sum n^{-2}x^2∑n−2x2 converges in [-1/4, 1/3]