# Talk:Quantum field theory

## Definition of ${\rho_{\kappa}}(\Box)$
Could someone please explain the meaning of ${\rho_{\kappa}}(\Box)$, which occurs two displayed equations below Equation (6)? I checked the published version of Encyclopedia of Mathematics to see if the expression appears there as well, and indeed it does. However, no definition is given.
Probably, the function $\rho_{\kappa}$ applied to the d'Alembertian $\Box$. In the formula for the smoothing function $D_{\kappa,\epsilon}$ we see $\rho_{\kappa}(\langle p,p \rangle)$; and this Fourier transform diagonalizes $\Box$, turning it into multiplication by $p\mapsto\langle p,p \rangle$, right? Boris Tsirelson (talk) 16:15, 5 December 2016 (CET)
Hello Professor Tsirelson. Are you referring to the use of the Borel functional calculus to define ${\rho_{\kappa}}(\Box)$? This would make perfect sense because $\Box$ is a self-adjoint densely-defined operator on ${L^{2}}(\mathbb{R}^{4})$. Leonard Huang (talk) 19:57, 5 December 2016 (CET)