functions.py 文件源码

def powm1(ctx, x, y):
    mag = ctx.mag
    one = ctx.one
    w = x**y - one
    M = mag(w)
    # Only moderate cancellation
    if M > -8:
        return w
    # Check for the only possible exact cases
    if not w:
        if (not y) or (x in (1, -1, 1j, -1j) and ctx.isint(y)):
            return w
    x1 = x - one
    magy = mag(y)
    lnx = ctx.ln(x)
    # Small y: x^y - 1 ~ log(x)*y + O(log(x)^2 * y^2)
    if magy + mag(lnx) < -ctx.prec:
        return lnx*y + (lnx*y)**2/2
    # TODO: accurately eval the smaller of the real/imag part
    return ctx.sum_accurately(lambda: iter([x**y, -1]), 1)