Structure of the set of sums of a conditionally convergent series in a normed space

Conditions are investigated for the set of sums of a conditionally convergent series with terms in a normed space to be linear. Main result: if $\sum a_k$ is a conditionally convergent series such that $\sum a_kr_k(s)$ converges for almost all $s$, then the set of sums of the series $\sum a_k$ is linear ($(r_k)$ is the sequence of Rademacher functions).
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