isEffective -- Check if a divisor is effective

i1 : R = ZZ/31[x, y, u, v] / ideal(x * y - u * v)

o1 = R

o1 : QuotientRing

i2 : D1 = divisor({1, -2, 3, -4}, {ideal(x, u), ideal(x, v), ideal(y, u), ideal(y, v)})

o2 = 3*Div(y, u) + -4*Div(y, v) + 1*Div(x, u) + -2*Div(x, v) of R

o2 : WDiv

i3 : D2 = divisor({1, 39, 5, 27}, {ideal(x, v), ideal(y, v), ideal(x, u), ideal(x, u)})

o3 = 32*Div(x, u) + 39*Div(y, v) + 1*Div(x, v) of R

o3 : WDiv

i4 : isEffective( D1 )

o4 = false

i5 : isEffective( D2 )

o5 = true