??-symmetry BaGe3 superconductor Mathieu functions Meissner effect PT symmetry Research & information: general associative memory attractor complex Ginzburg–Landau equation cross-Kerr nonlinearity entanglement fermion quartets four-fermion attraction negativity nonlinear oscillator nonlinearly coupled oscillators open system quantum control quantum entanglement quantum machine learning quantum properties soliton stability analysis superconductivity s–wave symmetry Eliashberg formalism thermodynamic properties time-dependent driving fields