Abstract:
Several theorems on series of systems $\{\varphi(nx)\}$ are proved. It is shown, in particular, that there exists a continuous function $\varphi(x)$ such that a series of the system $\{\varphi(2^nx)\}$ with coefficients in $l_2$ does not converge in measure on $[0,1]$. This provides the answer to a problem raised by P. L. Ul'yanov.