 Quasi-Newton secant equation

For $p_k=x_{k+1}-x_k$ and $y_k=\nabla f(x_{k+1})-\nabla f(x_k)$, the Quasi-Newton secant equation requires $B_{k+1}{p}_k=y_k$. It reproduces the observed change of gradient along the latest step. If $B_{k+1}$ is a positive-definite symmetric matrix and $p_k\ne 0$, necessarily $p_k^T{y}_k=p_k^T{B}_{k+1}{p}_k>0$. A line search satisfying the weak Wolfe conditions ensures this: with $p_k={\alpha }_k{s}_k$,This necessary curvature condition is also sufficient for the existence of some positive-definite symmetric matrix mapping $p_k$ to $y_k$. In an orthonormal basis whose first vector is $p_k/\Vert p_k\Vert$, its first column must be $(a,b{)}^T=y_k/\Vert p_k\Vert$, with $a=p_k^T{y}_k/\Vert p_k{\Vert }^2>0$. The block matrix with first row $(a,b^T)$ and lower-right block $b{b}^T/a+I$ has a positive Schur complement, so is positive-definite.