φ2 inflation at its endpoint

In the simplest inflationary model V = 1/2 m(2)phi(2), we provide a prediction accurate up to 1% for the spectral index n(s) and the tensor-to-scalar ratio r assuming instantaneous reheating and a standard thermal history: n(s) = 0.9668 +/- 0.0003 and r = 0.131 +/- 0.001. This represents the simplest and most informative point in the (n(s), r) plane. The result is independent of the details of reheating (or preheating) provided the conversion to radiation is sufficiently fast. A slower reheating or a modified postinflationary evolution (with an equation of state parameter w <= 1/3) pushes towards smaller n(s) (and larger r), so that our prediction corresponds to the maximum n(s) (and minimum r) for the quadratic potential. We also derive similar results for a general V proportional to phi(p) potential.