Complex Substitution and Infinity in Quantum Mechanics Integrals

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SUMMARY

The discussion centers on solving integrals in Griffiths' "Introduction to Quantum Mechanics," specifically problems 2.22 and 6.7, involving complex substitutions. The integral limits originally span from negative infinity to positive infinity, and the substitution introduces a complex constant. It is established that if the function remains well-behaved at infinity and there are no poles between the original and new lines, the limits after substitution remain negative infinity to positive infinity, supported by the Cauchy Integral theorem.

PREREQUISITES
  • Understanding of complex analysis concepts, particularly the Cauchy Integral theorem.
  • Familiarity with quantum mechanics principles as outlined in Griffiths' textbook.
  • Knowledge of integral calculus, specifically techniques for substitution in integrals.
  • Basic understanding of limits and behavior of functions at infinity.
NEXT STEPS
  • Study the Cauchy Integral theorem in detail to understand its application in complex analysis.
  • Review Griffiths' "Introduction to Quantum Mechanics" for deeper insights into the specific problems discussed.
  • Explore techniques for evaluating integrals with complex substitutions.
  • Learn about the behavior of functions at infinity and conditions for well-behaved functions.
USEFUL FOR

Students and researchers in quantum mechanics, particularly those tackling integrals involving complex variables and substitutions, as well as individuals interested in the intersection of quantum mechanics and complex analysis.

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Homework Statement


In Griffiths' Introduction to Quantum Mechanics problem 2.22 as well as 6.7, I used substitution to complete an integral. The original integral had limits from negative infinity to positive infinity. For my substitution, I had a complex constant term added to the original variable. In computing the new limits of the integral after substitution, I must somehow add a complex number to infinity. Does this imply that the new limits are also negative infinity to infinity? Also, I haven't had analysis nor complex analysis and so I am unsure as to how to appropriately phrase what is going on in computing these new limits.


Homework Equations





The Attempt at a Solution

 
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The new integral is over a line shifted by a complex constant from the original line. IF there are no poles in the function between the original line and new line and IF the functions are 'well behaved' at infinity, i.e. go to zero fast enough, then you can argue using the Cauchy Integral theorem that the integral over both lines are equal. Given this is a quantum mechanics problem and not a complex analysis problem I suspect both IF's are probably true. So, yes, you can do that.
 

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