Power function on ℂ, ℝ, ℝ≥0, and ennreal
We construct the power functions x ^ y where
xandyare complex numbers,- or
xandyare real numbers, - or
xis a nonnegative real number andyis a real number; - or
xis a number from[0, +∞](a.k.a.ennreal) andyis a real number.
We also prove basic properties of these functions.
Equations
The real power function x^y, defined as the real part of the complex power function.
For x > 0, it is equal to exp(y log x). For x = 0, one sets 0^0=1 and 0^y=0 for y ≠ 0.
For x < 0, the definition is somewhat arbitary as it depends on the choice of a complex
determination of the logarithm. With our conventions, it is equal to exp (y log x) cos (πy).
For 0 ≤ x, the only problematic case in the equality x ^ y * x ^ z = x ^ (y + z) is for
x = 0 and y + z = 0, where the right hand side is 1 while the left hand side can vanish.
The inequality is always true, though, and given in this lemma.
real.rpow is continuous at all points except for the lower half of the y-axis.
In other words, the function λp:ℝ×ℝ, p.1^p.2 is continuous at (x, y) if x ≠ 0 or y > 0.
Multiple forms of the claim is provided in the current section.
Equations
The real power function x^y on extended nonnegative reals, defined for x : ennreal and
y : ℝ as the restriction of the real power function if 0 < x < ⊤, and with the natural values
for 0 and ⊤ (i.e., 0 ^ x = 0 for x > 0, 1 for x = 0 and ⊤ for x < 0, and
⊤ ^ x = 1 / 0 ^ x).