In Weinberg's book The Quantum Theory of Fields, Volume 1 on p.388 (Chapter 9), the following identity is used (with f being any "reasonable" function):
f(+∞)+f(−∞)=lim
I don't understand the identity in a qualitative / heuristic way.
Answer
I can understand the identity qualitatively now:
If the function f(t) has a well defined value at t = \pm\infty, then for large values of |t| the function f is essentially constant with values f(-\infty) for t < 0 and f(+\infty) for t > 0.
For tiny \epsilon the Exponential factor e^{-\epsilon |t|} is essentially equal to 1 for |t| < 1 / \epsilon , and almost Zero for large |t| > 1 / \epsilon.
We then have approximately: \epsilon\int_{-\infty}^\infty \mathrm{d}t \, f(t) e^{-\epsilon |t|} \approx \epsilon\int_{1 / \epsilon}^0 \mathrm{d}t \, f(-\infty) + \epsilon\int_0^{1 / \epsilon} \mathrm{d}t \, f(+\infty) = f(-\infty) + f(+\infty)
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