On some family of congruences of null strings
Ivor Robinson, Krzysztof Rózga · Journal of Mathematical Physics · 1984
A definition of a special family of congruences of null strings is given, and a link between its existence and ‘‘projective extensions of heavens’’ is provided. Next the integrability conditions of the problem are studied, and a subclass of all ‘‘heavens’’ admitting these special congruences is described in terms of one function of three variables subject to a nonlinear, differential second-order constraint.