Topic summary
Inherently serial problem

Extracted from the Wikipedia article Parallel algorithm.
Parallelizability
Some problems cannot be split up into parallel portions, as they require the results from a preceding step to effectively carry on with the next step – these are called inherently serial problems. Examples include iterative numerical methods, such as Newton's method, iterative solutions to the three-body problem, and most of the available algorithms to compute pi (π). Some sequential algorithms can be converted into parallel algorithms using automatic parallelization.