2026-07-072026-07-07http://salesiana.dossiersoluciones.com/handle/123456789/197589The gauge invariant observables of the closed bosonic string are quantized without anomalies in four space-time dimensions by constructing their quantum algebra in a manifestly covariant approach. The quantum algebra is the kernel of a derivation on the universal envelopping algebra of an infinite-dimensional Lie algebra. The search for Hilbert space representations of this algebra is separated from its construction, and postponed.22 pages. Revised: minor changes as in the published version (CMP)Mathematical PhysicsHigh Energy Physics - TheoryQuantum Physics81R10; 81T30text