Isotropic extension of first-order wave equations

The anisotropy of many one-dimensional and first-order-in-time (T$^1$) scalar wave equations (e.g., Korteweg-de Vries and Camassa-Holm) limits their physical completeness and applicability to bidirectional/high-dimensional systems. We define the T$^n\Lambda^m$ isotropic extension consisting of temporal order elevation and spatial tensorization, which is the only possible approach to eliminate anisotropy while preserving original solutions. Our analysis finds that the Burgers equation exhibits T$^{\mathbb{N}_+}\Lambda^{2\mathbb{N}+1}$ extensibility and the Korteweg-de Vries (KdV) equation exhibits the T$^{2\mathbb{N}_+}\Lambda^{2\mathbb{N}}$ extensibility. The T$^2\Lambda^0$ extension of the KdV equation leads to the corresponding isotropic T$^2$ equation (KdV$^2$) for shallow water dynamics, which is physically more complete and suitable for 2D generalization. In addition to inheriting all KdV solutions and conservation laws, the KdV$^2$ equation also provides linearly stable corrections to the Boussinesq equation. In contrast, the KdV-Burgers equation is inherently anisotropic as it fails to exhibit any T$^n\Lambda^m$ extensibility.