# Series expansion is separately expanding numerator and denominator

I am trying to expand a function in power series, but Mathematica is expanding the numerator and the denominator separately.

Series[-((
0.0435275 (18 - 20 Cos[ky] + 2 Cos[2 ky]))/((1 -
Cos[ky])^0.2 (3.03143 (1 - Cos[ky])^1.6 + Sin[ky]^2)^(
3/2))), {ky, 0, 6}]


The output is

SeriesData[ky, 0, {-0.2999998058828007, 0, -0.029999980588280066, 0,
0.006999995470598682}, 2, 7, 1]/((ky^2)^0.2 ((
SeriesData[ky, 0, {1, 0,
Rational[-1, 3], 0,
Rational[2, 45]}, 2, 7, 1]) + (ky^2)^1.6 (
SeriesData[
ky, 0, {0.9999989664885686, 0., -0.13333319553180914, 0.,
0.007777769739355534, 0., -0.00026454999113454205}, 0, 7,
1]))^(3/2))


How do I expand the function as a power series about ky = 0? Note: The function diverges at ky = 0, and ky is a real quantity.

• Use rational exponents, e.g., (1 - Cos[k y])^(8/5), and specify Assumptions -> ky >0 or Assumptions -> ky \[Element] Reals in the Series` function. Commented Apr 8, 2021 at 16:10
• @CarlWoll That works, please write it as an answer and I will accept it Commented Apr 8, 2021 at 16:49
• By the way, what is the best method to post such complicated output, which are hard to visualize after copy pasting (e.g. the function above)? Commented Apr 8, 2021 at 17:23