Generalisations and improvements of New Q-Newton's method Backtracking

In this paper, we propose a general framework for the algorithm New Q-Newton's method Backtracking, developed in the author's previous work. For a symmetric, square real matrix $A$, we define $minsp(A):=\min _{||e||=1} ||Ae||$. Given a $C^2$ cost function $f:\mathbb{R}^m\rightarrow \mathbb{R}$ and a real number $0}{||A(x)e_i(x)||}e_i(x);$$ (we can also normalise by $w(x)/\max \{1,||w(x)||\}$ when needed) $γ(x)>0$ learning rate chosen by Backtracking line search so that Armijo's condition is satisfied: $$f(x-γ(x)w(x))-f(x)\leq -\frac{1}{3}γ(x).$$ The update rule for our algorithm is $x\mapsto H(x)=x-γ(x)w(x)$. In New Q-Newton's method Backtracking, the choices are $τ=1+α>1$ and $e_1(x),\ldots ,e_m(x)$'s are eigenvectors of $\nabla ^2f(x)$. In this paper, we allow more flexibility and generality, for example $τ$ can be chosen to be $<1$ or $e_1(x),\ldots ,e_m(x)$'s are not necessarily eigenvectors of $\nabla ^2f(x)$. New Q-Newton's method Backtracking (as well as Backtracking gradient descent) is a special case, and some versions have flavours of quasi-Newton's methods. Several versions allow good theoretical guarantees. An application to solving systems of polynomial equations is given.

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