Title: On the global convergence of the Jacobi method for symmetric matrices of order 4 under parallel strategies,
Abstract: The paper analyzes special cyclic Jacobi methods for symmetric matrices of
order $4$. Only those cyclic pivot strategies that enable full parallelization
of the method are considered. These strategies, unlike the serial pivot
strategies, can force the method to be very slow or very fast within one cycle,
depending on the underlying matrix. Hence, for the global convergence proof one
has to consider two or three adjacent cycles. It is proved that for any
symmetric matrix $A$ of order~$4$ the inequality
$S(A^{[2]})\leq(1-10^{-5})S(A)$ holds, where $A^{[2]}$ results from $A$ by
applying two cycles of a particular parallel method. Here $S(A)$ stands for the
Frobenius norm of the strictly upper-triangular part of $A$. The result holds
for two special parallel strategies and implies the global convergence of the
method under all possible fully parallel strategies. It is also proved that for
every $\epsilon>0$ and $n\geq4$ there exist a symmetric matrix $A(\epsilon)$ of
order $n$ and a cyclic strategy, such that upon completion of the first cycle
of the appropriate Jacobi method the inequality $S(A^{[1]})>
(1-\epsilon)S(A(\epsilon))$ holds.