41. Correlation functions describe how microscopic variables, such as spin and density, at different positions are related.42. Where h ( r ) and c ( r ) are the indirect and direct correlation functions respectively. 43. Thus a symmetry in the Hamiltonian becomes a symmetry of the correlation function ( and vice versa ). 44. Asymptotics of correlation functions ( even for space, time and temperature dependence ) were evaluated in 1991. 45. Correlation functions of local operators living on the boundary of AdS are the natural analogues of scattering amplitudes.46. This allows, for example, the partition function to be used as a generating function for correlation functions . 47. A more general definition for photon antibunching concerns the slope of the correlation function away from zero time delay. 48. The second, the indirect correlation function , accounts for the effects of a third particle in a system. 49. Schwinger's foundational work on quantum field theory constructed the modern framework of field correlation functions and their anomalies. 50. This is discussed below in the section on the temporal evolution of correlation functions and Onsager's regression hypothesis. 