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The Napierian logarithm, as first defined by John Napier, is a function which can be defined in terms of the modern logarithm by:
(Being a quotient of logarithms, the base of the logarithm chosen is irrelevant.)
It is not a logarithm to any particular base in the modern sense of the term, however, it can be rewritten as:
and hence it is a linear function of a particular logarithm, and so satisfies identities quite similar to the modern one.