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**11 - 16**of**16**### 1 The concept of vacuum in quantum electrodynamics

"... A re-evaluation of the concept of vacuum in quantum electrodynamics is presented, focusing on the vacuum of the quantized electromagnetic field. In contrast to the ‘nothingness ’ associated to the idea of classical vacuum, subtle aspects are found in relation to the vacuum of the quantized electroma ..."

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A re-evaluation of the concept of vacuum in quantum electrodynamics is presented, focusing on the vacuum of the quantized electromagnetic field. In contrast to the ‘nothingness ’ associated to the idea of classical vacuum, subtle aspects are found in relation to the vacuum of the quantized electromagnetic field both at theoretical and experimental levels. These are not the usually called vacuum effects. The view defended here is that the so-called vacuum effects are not due to the ground state of the quantized electromagnetic field.

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"... Abstract: Nonperturbative effects in string theory are usually associated to D–branes. In many cases it can be explicitly shown that D–brane instantons control the large–order behavior of string perturbation theory, leading to the well–known (2g)! growth of the genus expansion. This paper presents a ..."

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Abstract: Nonperturbative effects in string theory are usually associated to D–branes. In many cases it can be explicitly shown that D–brane instantons control the large–order behavior of string perturbation theory, leading to the well–known (2g)! growth of the genus expansion. This paper presents a detailed treatment of nonperturbative solutions in string theory, and their relation to the large–order behavior of perturbation theory, making use of transseries and resurgent analysis. These are powerful techniques addressing general nonperturbative contributions within non–linear systems, which are developed at length herein as they apply to string theory. The cases of topological strings, the Painleve ́ I equation describing 2d quantum gravity, and the quartic matrix model, are explicitly addressed. These results generalize to minimal strings and general matrix models. It is shown that, in order to completely understand string theory at a fully nonperturbative level, new sectors are required beyond the standard D–brane sector.

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"... Preprint typeset in JHEP style- PAPER VERSION Lectures on non-perturbative effects in large N gauge ..."

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Preprint typeset in JHEP style- PAPER VERSION Lectures on non-perturbative effects in large N gauge

### Preprint typeset in JHEP style- PAPER VERSION The spectral problem of the ABJ Fermi gas

"... Abstract: The partition function on the three-sphere of ABJ theory can be rewritten into a partition function of a non-interacting Fermi gas, with an accompanying one-particle Hamilto-nian. We study the spectral problem defined by this Hamiltonian. We determine the exact WKB quantization condition, ..."

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Abstract: The partition function on the three-sphere of ABJ theory can be rewritten into a partition function of a non-interacting Fermi gas, with an accompanying one-particle Hamilto-nian. We study the spectral problem defined by this Hamiltonian. We determine the exact WKB quantization condition, which involves quantities from refined topological string theory, and test it successfully against numerical calculations of the spectrum. ar X iv