TITLE: What is a constant field?
QUESTION [5 upvotes]: I am looking at the following: 

 
Could you explain to me what a constant field is? 
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P.S. I found this in the paper of T. Honda, "Algebraic differential equation" (pages 170-176).

REPLY [3 votes]: The field of constants of a differential field is the subfield of elements a with $∂a=0$, see here.