Why are cumulants extensive and moments are not?

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In summary, extensive properties are important because they allow for accurate descriptions and estimates of large quantum systems. This is because 1-particle operators and the log of the expectation of an exponential are both additive for disjoint systems. The concept of size-extensivity is essential for the applicability of quantum estimates to large systems, even noninteracting ones. For further explanation, the article "Reflections on size-extensivity, size-consistency and generalized extensivity in many-body theory" by Nooijen, Shamasundar, and Mukherjee provides a good discussion. Additionally, the online book "Classical and Quantum Mechanics via Lie algebras" by Neumaier and Westra offers background information on this topic.
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annaphys
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And why does it matter if they are extensive or not?
 
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They are extensive because 1-particle operators are additive when applied to disjoint systems, and the log of the expectation of an exponential (which defines the cumulants from the partition function) is additive, too.

Size-extensivity is a necessary condition for the applicability of quantum estimates to large quantum systems, since without it not even a noninteracting large system is described correctly.
 
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That makes sense thank you. Do you have any resources on this that explains it further?
 
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A good discussion is in
  • Nooijen, Shamasundar & Mukherjee, Reflections on size-extensivity, size-consistency and generalized extensivity in many-body theory. Molecular Physics, 103 (2005), 2277-2298.
For the general background you might also benefit from reading Part II (Statistical mechanics) of my online book
 
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1. What are cumulants and moments?

Cumulants and moments are statistical measures used to describe the shape and distribution of a set of data. Moments are calculated using the raw data values, while cumulants are calculated using the moments.

2. Why are cumulants extensive?

Cumulants are extensive because they scale linearly with the size of the data set. This means that as the number of data points increases, the cumulants also increase proportionally.

3. Why are moments not extensive?

Moments are not extensive because they are affected by the scale of the data set. As the number of data points increases, the moments may not increase proportionally, and can even approach a limit or converge to a specific value.

4. What is the significance of extensivity in cumulants and moments?

Extensivity in cumulants and moments is important because it allows for a more accurate and consistent comparison of data sets of different sizes. By using extensive measures, we can better understand the underlying distribution of the data and make meaningful comparisons.

5. How are cumulants and moments used in scientific research?

Cumulants and moments are commonly used in fields such as physics, chemistry, and economics to analyze and compare data sets. They can provide valuable insights into the properties and behavior of complex systems, and are often used in statistical modeling and forecasting.

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