In this paper, motivated by the analysis of the fractional Laplace equation on the unit disk in $$\mathbb {R}^{2}$$
R
2
, we establish a characterisation of the weighted Sobolev space $$H_{\gamma }^{s}(\Omega )$$
H
γ
s
(
Ω
)
in terms of the decay rate of Fourier–Jacobi coefficients. This framework is then used to give a precise analysis of the solution to the fractional Laplace equation on the unit disk.