Limit of large but finite

In summary, the conversation focuses on the calculation of the eigenstates of the Hubbard Hamiltonian for two fermions. The ground state is given by (U2 - (U2 + 16t2)1/2)/2. It is noted that for U = infty, the result is 0, but this is an indeterminate form. For U >> t, the exchange energy J = -4t2/U is expected, and the question arises on how to derive J from the ground state equation. A Taylor series expansion is suggested as a solution. The conversation also briefly touches on the topic of moving the thread to a different forum.
  • #1
supernano
6
0
I've calculated the eigenstates of the Hubbard Hamiltonian for two fermions.
The ground state is (U2 - (U2 + 16t2)1/2)/2
For U = infty, I get 0.
For U >> t, I should get the exchange energy J = -4t2/U
How do I get from the ground state equation to J?
 
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  • #2
Use the Taylor series expansion ##\sqrt{1 + x} \approx 1 + x/2## for small x.
 
  • #3
Too easy, thanks!
 
  • #4
When I saw a thread called "limit of large but finite" under "calculus" I wasn't expecting something about the Hubbard Hamiltonian of two fermions. But, clearly, @DrClaude was prepared for anything!
 
  • #5
PeroK said:
When I saw a thread called "limit of large but finite" under "calculus" I wasn't expecting something about the Hubbard Hamiltonian of two fermions. But, clearly, @DrClaude was prepared for anything!
I was actually more prepared for the Hubbard Hamiltonian than for regular calculus :wink:

I hesitated to move the thread. @supernano, note that while your question was a mathematical one, you have a higher probability of getting an answer for such a question in the QM forum. It's more a question of knowing the tricks than knowing maths.
 
  • #6
supernano said:
I've calculated the eigenstates of the Hubbard Hamiltonian for two fermions.
The ground state is (U2 - (U2 + 16t2)1/2)/2
For U = infty, I get 0.
For one thing, you can't just substitute ##\infty## into the expression. For another thing, the expression above has the form ##[\infty - \infty]##, which is one of several indeterminate forms, along with ##[\frac0 0]##, ##[\frac{\infty}{\infty}]## and a few others.
supernano said:
For U >> t, I should get the exchange energy J = -4t2/U
How do I get from the ground state equation to J?
 

Related to Limit of large but finite

1. What is the definition of a limit of large but finite?

The limit of large but finite refers to the value that a function approaches as the input approaches a very large number, but remains finite. It is a way to describe the behavior of a function as the input gets closer and closer to infinity.

2. How is the limit of large but finite different from the limit at infinity?

The limit of large but finite is a specific type of limit where the input approaches infinity but remains finite. The limit at infinity, on the other hand, refers to the behavior of a function as the input reaches infinity without any restrictions on its finiteness.

3. Can the limit of large but finite be infinite?

No, the limit of large but finite cannot be infinite. As the input approaches infinity, the value of the function may become very large, but it will still be a finite value. The limit at infinity, however, can be infinite if the function grows without bound as the input approaches infinity.

4. How do you calculate the limit of large but finite?

The limit of large but finite can be calculated using the same methods as any other limit. This includes substitution, factoring, and using algebraic manipulations. However, it is important to consider the behavior of the function as the input approaches infinity, and whether it remains finite or approaches infinity itself.

5. Why is the limit of large but finite important in mathematics and science?

The limit of large but finite is important because it helps us understand the behavior of functions as they approach very large numbers. This can be useful in modeling real-world phenomena, such as population growth, where the input (time) can become very large, but the population size remains finite. It also has applications in calculus, where it is used to find the area under curves and calculate rates of change.

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