Approximate Identity: Functional analysis, Banach algebra, Net (mathematics), Mollifier, Dirac delta function, Fejér kernel, Fourier series, C* Convolution

Analytic Semigroup: Mathematics, C0 semigroup, Partial differential equation, Smooth function, Spectrum (functional...

Linear Response Function: Susceptibility, Electrical impedance, Fundamental solution, Volterra series, Harmonic oscillator,...

Dirac Comb: Mathematics, Electrical engineering, Periodic function, Schwartz distribution, delta Ronald N. Bracewell

Isolated Singularity: Complex Analysis, Mathematics, Functional Holomorphic Function, Mathematical Singularity.