A Generalisation of Goursat's Algorithm for Integration in Finite Terms
For mathematicians studying integration in finite terms, this provides a novel extension of classical results to a new class of integrals, though the work is primarily theoretical and incremental.
The paper generalizes Goursat's 1887 theorem on pseudo-elliptic integrals to cube-root integrals of the form ∫F(t)dt/∛R(t) with R cubic, showing that two of three eigenpieces yield elementary antiderivatives while the third is generically transcendental.
We give a self-contained, modern exposition of Édouard Goursat's 1887 theorem on pseudo-elliptic integrals -- those integrals of the form $\int F(t)\,\d t/\sqrt{R(t)}$ with $R$ a cubic or quartic polynomial that, despite living on a genus-$1$ algebraic curve, admit elementary antiderivatives. After reviewing integration in finite terms and Liouville's theorem, we present Goursat's two main theorems with proofs phrased in the language of Möbius automorphisms of the underlying hyperelliptic curve. We then develop a cube-root analog: for integrals of the form $\int F(t)\,\d t/\sqrt[3]{R(t)}$ with $R$ cubic, an order-$3$ Möbius substitution cyclically permuting the roots of $R$ induces an eigendecomposition into three pieces. Two of the three eigenpieces (eigenvalues $1$ and $ω^2$, where $ω= e^{2πi/3}$) descend through a chain of substitutions to genus-$0$ curves and yield elementary antiderivatives; the middle eigenpiece (eigenvalue $ω$) descends only to the genus-$1$ curve $y^3 = x(x-K)$ and is generically transcendental.