DeRhamComputation/superelliptic/superelliptic_function_class.sage

class superelliptic_function:
    '''Class of rational functions on a superelliptic curve C. g = g(x, y) is a polynomial
       defining the function.'''
    def __init__(self, C, g):
        F = C.base_ring
        Rxy.<x, y> = PolynomialRing(F, 2)
        Fxy = FractionField(Rxy)
        f = C.polynomial
        r = f.degree()
        m = C.exponent
        
        self.curve = C
        g = reduction(C, g)
        self.function = g
    
    def __eq__(self, other):
        if self.function == other.function:
            return True
        return False
    
    def __repr__(self):
        return str(self.function)
    
    def jth_component(self, j):
        g = self.function
        C = self.curve
        F = C.base_ring
        Rx.<x> = PolynomialRing(F)
        Fx.<x> = FractionField(Rx)
        FxRy.<y> = PolynomialRing(Fx)
        g = FxRy(g)
        return coff(g, j)
    
    def __add__(self, other):
        C = self.curve
        g1 = self.function
        g2 = other.function
        g = reduction(C, g1 + g2)
        return superelliptic_function(C, g)
    
    def __neg__(self):
        C = self.curve
        g = self.function
        return superelliptic_function(C, -g)
    
    def __sub__(self, other):
        C = self.curve
        g1 = self.function
        g2 = other.function
        g = reduction(C, g1 - g2)
        return superelliptic_function(C, g)
    
    def __mul__(self, other):
        C = self.curve
        F = C.base_ring
        if isinstance(other, superelliptic_function):
            g1 = self.function
            g2 = other.function
            g = reduction(C, g1 * g2)
            return superelliptic_function(C, g)
        if isinstance(other, superelliptic_form):
            g1 = self.function
            g2 = other.form
            g = reduction(C, g1 * g2)
            return superelliptic_form(C, g)
        
    def __rmul__(self, constant):
        C = self.curve
        g = self.function
        return superelliptic_function(C, constant*g)
    
    def __truediv__(self, other):
        C = self.curve
        g1 = self.function
        g2 = other.function
        g = reduction(C, g1 / g2)
        return superelliptic_function(C, g)

    def __pow__(self, exp):
        C = self.curve
        g = self.function
        return superelliptic_function(C, g^(exp))
    
    def diffn(self):
        C = self.curve
        f = C.polynomial
        m = C.exponent
        F = C.base_ring
        g = self.function
        Rxy.<x, y> = PolynomialRing(F, 2)
        Fxy = FractionField(Rxy)
        g = Fxy(g)
        A = g.derivative(x)
        B = g.derivative(y)*f.derivative(x)/(m*y^(m-1))
        return superelliptic_form(C, A+B)

    def coordinates(self, basis = 0, basis_holo = 0, prec=50):
        '''Find coordinates in H1(X, OX) in given basis basis with dual basis basis_holo.'''
        C = self.curve
        if basis == 0:
            basis = C.cohomology_of_structure_sheaf_basis()
        if basis_holo == 0:
            basis_holo = C.holomorphic_differentials_basis()
        g = C.genus()
        coordinates = g*[0]
        for i, omega in enumerate(basis_holo):
            coordinates[i] = -omega.serre_duality_pairing(self, prec=prec)
        return coordinates
    
    def expansion_at_infty(self, place = 0, prec=20):
        C = self.curve
        fct = self.function
        F = C.base_ring
        Rt.<t> = LaurentSeriesRing(F, default_prec=prec)
        xx = C.x_series[place]
        yy = C.y_series[place]
        return Rt(fct(x = Rt(xx), y = Rt(yy)))
    
    def expansion(self, pt, prec = 50):
        '''Expansion in the completed ring of the point pt. If pt is an integer, it means the corresponding place at infinity.'''
        if pt in ZZ:
            return self.expansion_at_infty(place=pt, prec=prec)
        x0, y0 = pt[0], pt[1]
        C = self.curve
        f = C.polynomial
        F = C.base_ring
        m = C.exponent
        Rt.<t> = LaurentSeriesRing(F, default_prec=prec)
        Rxy.<x, y> = PolynomialRing(F, 2)
        Fxy = FractionField(Rxy)
        if y0 !=0 and f.derivative()(x0) != 0:
            y_series = f(x = t + x0).nth_root(m)
            return Rt(self.function(x = t + x0, y = y_series))
        if f.derivative()(x0) == 0: # then x - x0 is a uniformizer
            y_series = Rt(f(x = t+x0).nth_root(m))
            return Rt(self.function(x = t + x0, y = y_series))
        if y0 == 0: #then y is a uniformizer
            f1 = f(x = x+x0) - y0
            x_series = new_reverse(f1(x = t), prec = prec)
            x_series = x_series(t = t^m - y0) + x0
            return self.function(x = x_series,  y = t)

    def pth_root(self):
        '''Compute p-th root of given function. This uses the following fact: if h = H^p, then C(h*dx/x) = H*dx/x.'''
        C = self.curve
        if self.diffn().form != 0:
            raise ValueError("Function is not a p-th power.")
        Fxy, Rxy, x, y = C.fct_field
        auxilliary_form = superelliptic_form(C, self.function/x)
        auxilliary_form = auxilliary_form.cartier()
        auxilliary_form = C.x * auxilliary_form
        auxilliary_form = auxilliary_form.form
        return superelliptic_function(C, auxilliary_form)
    
    def valuation(self, place = 0):
        '''Return valuation at i-th place at infinity.'''
        C = self.curve
        F = C.base_ring
        Rt.<t> = LaurentSeriesRing(F)
        return Rt(self.expansion_at_infty(place = place)).valuation()