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1 Introduction
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2 Observations
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3 Characteristics of the extragalactic sources
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4 3.4 m absorption feature
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5 The 3.4 m hydrocarbons/10 m silicates ratio
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6 Galactic center and extragalactic mid infrared spectra
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7 Constraints on the nature of the hydrocarbons
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8 Conclusions
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Appendix: Integrated absorbances of ketones and alcanes at 10 K
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References
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