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I'm solving a linearized ODE (to study accretion flow in a disk) ar = NDSolve[{D[F[t, l]^-1*l^7, t] == D[D[F[t, l], l], l], F[t, 50] == 2, F[t, .001] == 0, F[0, l] == Sigma4[l]}, F, {t, 0, 1000}, {l, .001, 10}]; br= Plot3D[Evaluate[F[t,...