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We investigate the three-dimensional O(2) model near the critical point byMonte Carlo simulations and calculate the major universal amplitude ratios ofthe model. The ratio U_0=A+/A- is determined directly from the specific heatdata at zero magnetic field. The data do not, however, allow to extract anaccurate estimate for alpha. Instead, we establish a strong correlation of U_0with the value of alpha used in the fit. This numerical alpha-dependence isgiven by A+/A- = 1 -4.20(5) alpha + O(alpha^2). For the special alpha-valuesused in other calculations we find full agreement with the corresponding ratiovalues, e. g. that of the shuttle experiment with liquid helium. On thecritical isochore we obtain the ratio xi+/xi-_T=0.293(9), and on the criticalline the ratio xi_T^c/xi_L^c=1.957(10) for the amplitudes of the transverse andlongitudinal correlation lengths. These two ratios are independent of the usedalpha or nu-values.Comment: 34 pages, 19 Ps-figures, Latex2e, revised version, to be published in  J. Phys.

Universal amplitude ratios from numerical studies of the three-dimensional O(2) model