  Let K be a field and \tilde{K} denote the set of all r \in K for which there
exists a finite set A(r) with {r} \subseteq A(r) \subseteq K such that each
mapping f:A(r) \to K that satisfies: if 1 \in A(r) then f(1)=1, if a,b \in A(r)
and a+b \in A(r) then f(a+b)=f(a)+f(b), if a,b \in A(r) and a \cdot b \in A(r)
then f(a \cdot b)=f(a) \cdot f(b), satisfies also f(r)=r. We prove: \tilde{K}
is a subfield of K, \tilde{K}={x \in K: {x} is existentially first-order
definable in the language of rings without parameters}, if some subfield of K
is algebraically closed then \tilde{K} is the prime field in K, some elements
of \tilde{K} are transcendental over Q (over R, over Q_p) for a large class of
fields K that are finitely generated over Q (that extend R, that extend Q_p),
if K is a Pythagorean subfield of R, t is transcendental over K, and r \in K is
recursively approximable, then {r} is \emptyset-definable in
(K(t),+,\cdot,0,1), if a real number r is recursively approximable then {r} is
existentially \emptyset-definable in (R,+,\cdot,0,1,U) for some unary predicate
U which is implicitly \emptyset-definable in (R,+,\cdot,0,1).
