Three‐dimensional steady boundary layer equations are considered as a model case of a more general class of problems called ’’multitime initial‐value problem’’ for a parabolic system. Ideas developed in recent years for solving three‐dimensional steady boundary layers are generalized for application to other similar problems, such as unsteady two‐dimensional boundary layers. An unsteady counterpart of the wedge‐shaped zones of influence and dependence is defined and then used to rationalize the calculation of reversed flow in two‐dimensional unsteady case. Schemes used previously for the latter flow are re‐examined in light of this dependence rule, and it is found that these schemes could satisfy such a rule.

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