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Let $M$ be a Riemannian manifold with Levi-Civita connection $\nabla$ and $K$ a $(1,1)$ tensor field on $M$. I am trying to prove that if $K$ is parallel then $\nabla_X K(Y) = K(\nabla_XY)$ for every $X,Y$ vector fields on $M$.

I tried to use the definition of covariant derivative for tensor fields but it deals with the covariant derivative of $1$-forms, I mean, the formula is:

$$(\nabla_XK)(Y,\omega) = X(K(Y,\omega)) - K(\nabla_XY,\omega) - K(Y, \nabla_X \omega).$$

How can I conclude the claim?

Thanks

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    The formula you wrote is not for $(1,1)$ tensors but for $(1,2)$ tensors (under a certain identification). – 2017-01-22

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The formula for the covariant derivative of a $(1,1)$ tensor $K$ is

$$ (\nabla_X K)(Y) = \nabla_X(K(Y)) - K(\nabla_X Y). $$

If $K$ is parallel then $\nabla_X K = 0$ and so $\nabla_X(K(Y)) = K(\nabla_X Y)$ for all vector fields $Y$ on $M$.

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    Easy, thank you! I probably made some confusion! Thanks. – 2017-01-22