http://math.stackexchange.com/questions/1425676/gravitational-potential-inside-infinite-grid-of-masses

gravnet.log

n = 98: r = Vector[0.5, 0.5], F = Vector[-0.014357405607155703, -0.01435740560715777]
n = 99: r = Vector[0.5, 0.5], F = Vector[-0.014213111904535183, -0.014213111904536712]
n = 100: r = Vector[0.5, 0.5], F = Vector[-0.014071689664539606, -0.01407168966453928]

n = 98: r = Vector[0.25, 0.25], F = Vector[-4.6817100348218865, -4.6817100348219425]
n = 99: r = Vector[0.25, 0.25], F = Vector[-4.681637888660634, -4.681637888660691]
n = 100: r = Vector[0.25, 0.25], F = Vector[-4.681567178203503, -4.681567178203558]

n = 98: r = Vector[0.25, 0.0], F = Vector[-14.708761955833403, -1.0966027348333074e-16]
n = 99: r = Vector[0.25, 0.0], F = Vector[-14.7086898091545, 1.0235546134620965e-16]
n = 100: r = Vector[0.25, 0.0], F = Vector[-14.708619098200186, 9.540979117872439e-17]

n = 98: r = Vector[0.0, 0.25], F = Vector[2.872627537318234e-15, -14.708761955833696]
n = 99: r = Vector[0.0, 0.25], F = Vector[1.2367358656270588e-16, -14.708689809154844]
n = 100: r = Vector[0.0, 0.25], F = Vector[-4.756347496848862e-15, -14.708619098200511]