Complexity: are easily-checked problems also easily solved?
That’s Maths: Many problems are hard to solve but easy to verify, such as Sudoku puzzles
South Africa in control against Sri Lanka after Cook hits century
Tourists face uphill task to retrieve third test in Port Elizabeth
Seven-wicket Kagiso Rabada puts South Africa in command
England still in with a slim chance after rain-interrupted third day of Fourth Test
Alistair Cook leads fightback after Quinton de Kock flays England
Tourists miss chances to finish South Africa off at Centurion as Proteas post big score
England finally put the brakes on runaway South Africa
Stephen Cook scores debut on his debut at Centurion
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