  Professor Tibor \v{S}al\'at, at one of his seminars at Comenius University,
Bratislava, asked to study the influence of gaps of an integer sequence
A={a_1<a_2<...<a_n<...} on its exponent of convergence. The exponent of
convergence of A coincides with its upper exponential density. In this paper we
consider an extension of Professor \v{S}al\'at's question and we study the
influence of the sequence of ratios a_m/a_{m+1} and of the sequence
(a_{m+1}-a_m)/a_m on the upper and on the lower exponential densities of A.
