21. Decision problems become harder to answer or completely undecidable . 22. Philip K . Hooper proved in 1966 that the mortality problem is undecidable . 23. Earlier, Alfred Tarski proved elementary group theory undecidable . 24. Higher-order unification is known to be undecidable . 25. These are natural mathematical equivalents of the G�del " true but undecidable " sentence. 26. The list of undecidable problems gives additional examples of problems with no computable solution. 27. The equivalence of two lambda expressions is undecidable . 28. For more expressive formalisms, this problem can be harder, or even undecidable . 29. :A source of confusion may be that " undecidable " has several meanings. 30. Partially decidable problems and any other problems that are not decidable are called undecidable .
