On the Lefschetz Standard Conjecture

The subject of the present paper is Grothendieck's Lefschetz standard conjecture $B(X)$. Our main result is that, if $X$ is a projective smooth variety of dimension $n$ and the conjecture $B({\cal Y})$ holds for the generic fibre ${\cal Y}$ (of dimension $n-1$ over the field $k(t)$) of a suitable Lefschetz fibration of $X$, then the operator $\Lambda_X-p^{n+1}_X$ is algebraic. If in addition $p^{n+1}_X$ is algebraic, then $B(X)$ is settled. Along the way we establish the algebraicity of the K\" unneth projectors $\pi^i_X$ for $i\neq n-1, n, n+1$ under the above hypotheses.