“…[1, 4-6, 9-16, 20]), Boundary Element-Based Finite Element Methods (BEM-FEM) (cf. [26,[32][33][34][39][40][41][42][43][44]) and Trefftz-type Finite Element Methods (Trefftz-FEM) (cf. [2,3,18,24,25]) all employ vector spaces that are implicitly defined, as in (1).…”