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Cosmological models with coherent scalar field oscillations in fourth-order gravity

Spatially flat Friedman models driven at early times by a coherent scalar field and the vacuum polarisation are considered. Maxima and minima as well as inflationary expansion of the scale factor result from both effects. The possibility of a start of the cosmological expansion at a finite bounce is a welcome property, but in the flat model it leads to an unstable late-time behaviour.