[issue27761] Private _nth_root function loses accuracy

[Serhiy Storchaka]

Serhiy Storchaka added the comment:

Wouldn't following implementation be faster?

    import decimal
    c = decimal.DefaultContext.copy()
    c.prec = 25
    def rootn(x, n,
              D=decimal.Decimal,
              sub=c.subtract,
              mul=c.multiply,
              log=c.ln):
        g = x ** (1.0/n)
        g += float(sub(log(D(x)), mul(log(D(g)), D(n)))) * g / n
        return g
    del decimal, c

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