Published online by Cambridge University Press: 09 January 2018
Let   ${\cal F}$  =(F; +, .,0, 1, D) be a differentially closed field. We consider the question of definability of the derivation D in reducts of
 ${\cal F}$  =(F; +, .,0, 1, D) be a differentially closed field. We consider the question of definability of the derivation D in reducts of   ${\cal F}$  of the form
 ${\cal F}$  of the form   ${\cal F}$  R  = (F; +, .,0, 1, P)P ε R  where R is some collection of definable sets in
 ${\cal F}$  R  = (F; +, .,0, 1, P)P ε R  where R is some collection of definable sets in   ${\cal F}$ . We give examples and nonexamples and establish some criteria for definability of D. Finally, using the tools developed in the article, we prove that under the assumption of inductiveness of Th (
 ${\cal F}$ . We give examples and nonexamples and establish some criteria for definability of D. Finally, using the tools developed in the article, we prove that under the assumption of inductiveness of Th (  ${\cal F}$  R ) model completeness is a necessary condition for definability of D. This can be seen as part of a broader project where one is interested in finding Ax-Schanuel type inequalities (or predimension inequalities) for differential equations.
 ${\cal F}$  R ) model completeness is a necessary condition for definability of D. This can be seen as part of a broader project where one is interested in finding Ax-Schanuel type inequalities (or predimension inequalities) for differential equations.