EntityQ669094· pop 12· linked from 163 articlesarithmetical hierarchySign in to savehierarchy which classifies certain sets based on the complexity of formulas that define themConnectionsPeano axiomsEntityrecursive setEntityoracle machineEntityrecursively enumerable setEntityTuring reductionEntityGδ setEntityInternational Standard Book NumberEntitynatural numberEntitysetEntitydigital object identifierEntitymathematical logicEntityTuring machineEntityCartesian productEntitydivisorEntityunionEntitytopological spaceEntityintersectionEntityequivalence relationEntitycomplementEntityordinal numberEntityCategoriesComplexity classesComputability theoryEffective descriptive set theoryHierarchyMathematical logic hierarchies