Question on Cosmological Constant paper

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SUMMARY

The discussion centers on the evaluation of a sum in the context of the cosmological constant paper found at http://arxiv.org/pdf/1205.3365v1.pdf. The key argument is that as time approaches infinity with the substitution t = (1-iϵ)s, all terms in the sum S = ∑_n exp(-i E_n t) converge to zero except for the leading term associated with the lowest energy E_0. This is due to the rapid oscillation of the first term and the faster decay of subsequent terms, establishing that the dominant contribution to the sum is from the E_0 term.

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In the paper, http://arxiv.org/pdf/1205.3365v1.pdf, page 21, the author argues that if:
t →∞(1-iϵ), all the terms in equation (193) goes to zero, except the first term.

Can anyone explain this to me?

Thanks
 
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Well, it's pretty simple I think. Suppose you have a sum
S = \sum_n \exp(-i E_n t). Let t = (1-i \epsilon) s where s is real. Then S = \sum_n \exp(-i E_n s) \exp(-E_n s). Now you want to evaluate this sum when s \rightarrow \infty. In that limit, the first term oscillates rapidly and the second term of the sum goes to zero.

The dominating component of the sum is the one that goes to zero the slowest. If we order the E:s so that E_0 < E_1 < E_2 < ..., then the leading order behaviour is given by the E_0-term as the rest go to zero even faster than that one.
 
Thanks
 

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