We derive conditions for recurrence and transience for time-inhomogeneous birth-and-death processes considered as random walks with positively biased drifts. We establish a general result, from which the earlier known particular results by Menshikov and Volkov [‘Urn-related random walk with drift $\rho x^\alpha /t^\beta $’, Electron. J. Probab. 13 (2008), 944–960] follow.

This paper generalizes the Kunita–Watanabe decomposition of an $L^2$ space. The generalization comes from using nonlinear stochastic integrals where the integrator is a family of continuous martingales bounded in $L^2$. This result is also the solution of an optimization problem in $L^2$. First, martingales are assumed to be stochastic integrals. Then, to get the general result, it is shown that the regularity of the family of martingales with respect to its spatial parameter is inherited by the integrands in the integral representation of the martingales. Finally, an example showing how the results of this paper, with the Clark–Ocone formula, can be applied to polynomial functions of Brownian integrals.

Using an empirical approach to capital market returns analogous to that used for mortality rates by Halley more than three centuries ago to establish life assurance on a sound and scientific footing, a theory of option pricing is built up in terms of the same three key components as for life assurance premiums, namely the expected cost of claims, an allowance for expenses, and a contingency margin as a reserve against the risk of insolvency. The dimensionality of the process describing security returns to any future point in time is increased from two to three by the addition of systematic variability around ‘central values’ to the standard descriptors of expected return and variance of return. It is shown that this approach, which involves only common sense principles and elementary mathematics, has important theoretical, practical and regulatory advantages over the Black-Scholes and related methodologies of modern finance theory.

The Itô formula is the fundamental theorem of stochastic calculus. This short note presents a new proof of Itô's formula for the case of continuous semimartingales. The new proof is more geometric than previous approaches, and has the particular advantage of generalizing immediately to the multivariate case without extra notational complexity.