Logarithmic function
It may be shown, by Mathematical Induction, that the Geometric series has the indicated value:
1 + x + x^2 + x^3 + ... + x^i + ... + x^n = (1 - x^(n + 1)) / (1 - x)
The limit, as n increases without bound, is 1 / (1 - x) provided that x is inside of its circle of convergence: abs(x) < 1.
We define the logarithmic function as the inverse of the exponential function; hence, the derivative of the logarithmic function is d ln(x) / dx = 1 / x. Thus, the indefinite iintegral of 1/x is ln(abs(x)) + C. In passing, we observe that, for real a and b, the logarithm of the complex number a + ib may be expressed as ln(a + ib) = (1 / 2) ln(a^2 + b^2) + iArctan(b / a).
Let us integrate the infinite Geometric series to obtain:
- ln(1 - x) = x + x^2 / 2 + x^3 / 3 + x^4 / 4 + ... + x^i / i + .... provided abs(x) < 1
Replace x by its additive inverse (that is, by -x):
ln(1 + x) = x - x^2 / 2 + x^3 / 3 - x^4 / 4 + ... - (-x)^i / i + .... provided abs(x) < 1
Take half the sum:
(1 / 2) ln((1 + x) / (1 - x)) = x + x^3 / 3 + x^5 / 5 + x ^7 / 7 + ... + x^(2i + 1) / (2i + 1) + .... provided abs(x) < 1
Let y = (1 + x) / (1 - x); then x = (y + 1) / (y - 1). This transformation is called "bilinear"; because it is the quotient of two linear expressions. It has many interesting properties: for instance, it maps conic sections into conic sections. We may employ this transformation in the foregoing to obtain the logarithm of y provided that the real-part of y is strictly positive, which we write Re
> 0:
ln
= 2 x (1 + x^2 / 3 + x^4 / 5 + x^6 / 7 + ... + x^(2i) / (2i + 1) + ....) provided Re
> 0
Of course, we would square x once, then employ that value in the series. The ratio of the i-th term to the (i- 1) term is x^2 2i / (2i + 1). See the evaluation of a power series, for a practical method of evaluation of a power series.
In passing, we observe that the logarithmic function has an essential singularity at the origin.That means that the limit of the logarithmic function, as x approaches zero, depends upon the angular direction of the approach.