The article contains sections titled: 1. Introduction 2. Ketene 2.1. Physical Properties 2.2. Chemical Properties 2.3. Production 2.4. Uses 3. Diketene 3.1. Physical Properties 3.2. Chemical Properties 3.3. Production 3.4. Storage, Handling, and Transportation 3.5. Uses 3.5.1. First‐Generation Diketene Derivatives 3.5.2. Second‐Generation Diketene Derivatives 4. Higher Ketenes 4.1. Properties 4.2. Production and Uses 5. Toxicology and Occupational Health

We prove that the asymptotic variance of a drift accelerated diffusion converges to zero uniformly if and only if there are no H 1 -elements in the kernel of the drift generating operator. Our proof is based on spectral analysis in the first order Sobolev space of mean zero functions.

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