inverse(s)
i1 : R=ZZ[x,y] o1 = R o1 : PolynomialRing
i2 : s = series(2*x^2*y+x*y+x^2+x+y+1,3) 2 2 o2 = FormalSeries{2x y + x + x*y + x + y + 1, 3} o2 : FormalSeries
i3 : inverse(s) 3 2 2 3 2 o3 = FormalSeries{x - x y - x*y - y + x*y + y - x - y + 1, 3} o3 : FormalSeries
If the constant coefficient of s is not +1 or -1, inverse returns an error.