# Math Help: Commutator & Relation [f(\hat{A}),\hat{B}]

• noospace
In summary, a commutator in math is a measure of the non-commutativity between two operators. It is denoted by [A, B] and can be calculated using the formula [A, B] = AB - BA. The commutator [f(A), B] represents the non-commutativity between f(A) and B. If the commutator is zero, the operators commute and the order of operations does not matter. If it is non-zero, the operators do not commute and the order of operations does matter. The commutator is significant in understanding non-commutativity in mathematics and is also important in quantum mechanics.
noospace
Does the relation $[f(\hat{A}),\hat{B}] = df(\hat{A})/d\hat{A}$ follow when A commutes with [A,B]? or is this only valid when [A,B]=1?

If A commutes with [A,B] then:

$$[f(A),B] = [A,B] f'(A)$$

You should try to derive this formula. Use a taylor expansion of f(A), ie:

$$f(A) = f(0) + f'(0) A + \frac{1}{2} f''(0) A^2 + ...$$

## 1. What is a commutator in math?

A commutator in math is a mathematical operation that measures the extent to which two operators, represented by A and B, do not commute. In other words, it measures how much the order of the operations matters. The commutator is denoted by [A, B].

## 2. How is the commutator related to the operators f(A) and B?

The commutator [f(A), B] is a measure of the non-commutativity between the operator f(A) and the operator B. It represents the difference between the results of applying the operators in the order f(A)B and BA.

## 3. What does the commutator tell us about the operators f(A) and B?

The commutator [f(A), B] tells us about the non-commutativity of the operators. If the commutator is equal to zero, then the operators commute and the order of operations does not matter. If the commutator is non-zero, then the operators do not commute and the order of operations does matter.

## 4. How can the commutator be calculated?

The commutator [A, B] can be calculated using the formula [A, B] = AB - BA, where AB represents the result of applying the operators in the order AB and BA represents the result of applying the operators in the order BA. This formula can be extended to [f(A), B] by replacing A with f(A).

## 5. What is the significance of the commutator in math?

The commutator is significant in math because it helps us understand the non-commutativity of operators, which is a fundamental concept in mathematics. It is also important in quantum mechanics, where the commutator of two operators represents the uncertainty in measuring those operators simultaneously.

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