A kDelegating oracle with D i denoting the oracle EX i f i , X s i , D i. .. 6.2 An example of a mapping with errors and a semantics preserving mapping. 6.3 A schematic representation of reduction between various learning tasks. ix ACKNOWLEDGEMENTS When your graduate studies encompasses stints of two and four years spread over ten years, a marriage, two kids and three jobs it is no wonder that you have a lot of people to thank.