Multipoint Seshadri constants on P^2

Working over C and formalizing and sharpening approaches introduced in [12], [9] and [8], we give a method for verifying when a divisor on a blow up of ℙ2 at general points is nef. The method is useful both theoretically and when doing computer computations. The main application is to obtaining lower bounds on multipoint Seshadri constants on ℙ2. In combination with methods developed in [4], significantly improved explicit lower bounds are obtained.