Abstract:
A system of measurable functions $\{\varphi_k\}$ defined on a measurable space is called weakly multiplicative if it satisfies the relations
$$
\int_X\varphi_{k_1}\varphi_{k_2}\dots\varphi_{k_p}\,d\mu=0\quad(\forall p\geqslant2,\ k_1<k_2<\dots<k_p).
$$
In this paper the convergence in the metric of $L_p$ and a.e. is investigated for series of weakly multiplicative system of functions. One of the results is: {\it If $\{\varphi_k\}$ is weakly multiplicative and $\sup_k\|\varphi_k\|_p\leqslant M$ for some $p>2,$ then any series $\sum c_k\varphi_k$ with coefficients in $l_2$ converges unconditionally a.e. and in $L_p$}. For $p=2n$, instead of weak multiplicativity it is sufficient to require the condition $\int_X\varphi_{k_1}\dots\varphi_{k_{2n}}\,d\mu=0$ ($\forall k_1<\dots<k_{2n}$).
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