Doob's type optional sampling theorems and a central limit theorem for demimartingales with applications to associated sequences

Abstract: This paper extends classical probabilistic results to the broader class of demimartingales and demisubmartingales. We establish variants of Doob's-type optional sampling theorem under minimal structural conditions on stopping times, relying on monotonicity properties of indicator functions. Building on these foundations, we derive maximal inequalities, moment bounds, and a concentration inequality of Azuma- Hoeffding type for demimartingales with bounded increments. A central limit theorem and a strong law of large numbers are also obtained for demimartingales, demonstrating convergence under conditions considerably weaker than those required for martingales or independent sequences. These results are then applied to partial sums of positively associated random variables, yielding expectation bounds, concentration inequalities, and exponential bounds without requiring covariance decay or truncation arguments. The paper concludes with a strong law of large numbers for associated random variables, established under very mild conditions, highlighting the power of the demimartingale framework in handling dependence.