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

Introductory Mathematics: Algebra And Analysis (Springer Undergraduate Mathematics Series)

Linear Functional Analysis (Springer Undergraduate Mathematics Series)

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

Fourier Analysis on Groups (Dover Books Mathematics)