Nondegenerate Eignefunctions as Linear Combinations

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Discussion Overview

The discussion revolves around the properties of nondegenerate eigenfunctions and whether a linear combination of two eigenfunctions with different eigenvalues can yield another eigenfunction. The scope includes theoretical considerations in quantum mechanics and linear algebra.

Discussion Character

  • Exploratory, Technical explanation, Debate/contested

Main Points Raised

  • One participant asserts that linear combinations of eigenfunctions with the same eigenvalue yield another eigenfunction, but questions the case for different eigenvalues.
  • Another participant suggests that for the equation involving a linear combination to hold, the eigenvalues must be equal, implying that a linear combination of nondegenerate eigenfunctions cannot be an eigenfunction.
  • A subsequent reply confirms this view, stating that nondegenerate eigenfunctions are linearly independent, and thus a linear combination cannot represent another eigenfunction.

Areas of Agreement / Disagreement

Participants generally agree that a linear combination of nondegenerate eigenfunctions cannot yield another eigenfunction, but the discussion reflects a contestation of the implications and reasoning behind this conclusion.

Contextual Notes

The discussion does not explore potential exceptions or specific conditions under which the properties of eigenfunctions might differ, nor does it address any mathematical proofs or definitions that could clarify the claims made.

zachzach
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It is easily shown that two eigenfunctions with the same eigenvalues can be combined in a linear combination so that the linear combination is itself an eigenfunction. But what if the two eigenvalues are not the same? Can you still find a linear combination of the two functions that is an eigenfunction?

<br /> <br /> aE_1 \psi_1+ b E_2 \psi_2 = E(\psi_1 + \psi_2)<br /> <br />
 
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The equation you have written down you can set aE_1=bE_2 and it will satisfy the equation with E=aE1.

But what you're really looking for is:

<br /> <br /> <br /> aE_1 \psi_1+ b E_2 \psi_2 = E(a\psi_1 + b\psi_2)<br /> <br /> <br />

which can only be satisfied if E_1=E_2=E
 
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Yes, I meant to write the equation that you did. So the answer is no then. You can never have a linear combination of non degenerate eigenfunctions that is itself an eigenfunction.
 
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zachzach said:
Yes, I meant to write the equation that you did. So the answer is no then. You can never have a linear combination of non degenerate eigenfunctions that is itself an eigenfunction.

The idea is that the set of nondegenerate eigenfunctions is linearly independent. One eigenfunction cannot be the sum of two others, or else the set would not be linearly independent.
 

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