Anharmonic oscillator (quantum pendulum) is one of the oldest, non-solvable exactly problems in quantum physics. It can be shown that for the one-dimensional quartic quantum anharmonic oscillator with V (x) = x2 + g2x4 the Perturbation Theory (PT) in powers of g2 (weak coupling regime) and the semiclassical expansion in powers of ̄h for energies coincide. It is due to the fact that the dynamics in x-space and in (gx)-space corresponds to the same energy spectrum with effective coupling constant ( ̄hg2). Two equations, which govern the dynamics in those two spaces, the Riccati-Bloch (RB) and the Generalized Bloch (GB) equations, are derived. The PT in g2 for the log of wave function leads to polynomial in x coefficients at given order in g2 for the RB equation and to the true semiclassical expansion in powers of ̄h for the GB equation, which corresponds to a loop expansion for the density matrix in the path integral formalism. Matching these two expansions in a single function leads to a extremely compact, uniform approximation of the wavefunction in x-space with unprecedented accuracy ∼ 10−6 locally and unprecedented accuracy ∼ 10−9 −10−10 in energy for any g2 ≥ 0.