I am searching for a nonKilling vector field $\zeta \in\frak X\rm (M)$ where $(M,g)$ is a Riemannian manifold such that
$$\frac12 \frak L_\zeta \rm g+Ric=\lambda g$$
$$ \frak L_\zeta \rm Ric=\lambda \frak L_\zeta \rm g$$
The first equation is called soliton equation and $\zeta$ ia called a potential vector field. It is clear that if a Killing vector field is a solution of the first equation, then $(M,g)$ is Einstein with factor $\lambda$ and the second equation holds. I want to consider a nonKilling vector field. I checked 8 examples with nonconstant ricci curvature. In each case I got a system of difficult PDEs so I considered many special cases. All solutions are Killing vector fields.
My question
Is there an example with a nonKilling vector field satisfying these equations?
Or any hind to a nonexistence result.
Thanks in advance.
$\begingroup$
$\endgroup$

$\begingroup$ I made some edits thanks to Holonomia $\endgroup$ – Semsem Jan 9 '16 at 19:52
$\begingroup$
$\endgroup$
I don't know about general dimensions, but, for surfaces (i.e., when $M$ has dimension $2$), it is not hard to show, using the Cartan structure equations, that there is no solution to this system for which $\zeta$ is not a Killing field. It is an overdetermined system, and the compatibility conditions rapidly eliminate any possibility of there being a nontrivial solution in this case.

$\begingroup$ Thank you for your answer. Examples lead to the same conclusion for 4dimensional cases. But I don't know how to proceed. $\endgroup$ – Semsem Jan 9 '16 at 15:05

$\begingroup$ to Sameh Shenawy: if a Killing vector field is a solution your metric is Einstein, isn't it?. So a Killing vector field of a non Einstein manifold is not a solution. Perhaps I am misunderstanding something. $\endgroup$ – Holonomia Jan 9 '16 at 17:54

$\begingroup$ @Holonomia I think it should be as follows. The first part is: If $\zeta$ is Killing, then $M$ is Einstein with factor $\lambda$. Its equivalent is: If $M$ is not Einstein with factor $\lambda$, then $\zeta$ is not Killing. $\endgroup$ – Semsem Jan 9 '16 at 19:29

$\begingroup$ @Sameh Shenawy So your claim: "It is clear that a Killing vector field is a solution" is true just for Einstein manifolds and not in general for a Riemannian manifold as you wrote in your question. $\endgroup$ – Holonomia Jan 9 '16 at 19:42

$\begingroup$ @Holonomia yes you are right and i will edit my question now thanks to you $\endgroup$ – Semsem Jan 9 '16 at 19:43