# Steepest descent, method of

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A special instance of the method of descent (cf. Descent, method of), when the direction of descent is chosen as the direction opposite to . The formulas of the method of steepest descent are

where the parameters are determined by the condition that the function has maximum decrease at each step. If is twice continuously-differentiable and its matrix of second derivatives satisfies the inequality

for any , with constants , then (see [2], [4]) the sequence converges to a solution of the problem of minimizing , the convergence rate being that of a geometric progression with quotient .

The method of steepest descent has been widely applied to the solution of systems of linear algebraic equations with a Hermitian or positive-definite matrix . In the real symmetric case, the problem of solving this system is equivalent to finding a vector in an -dimensional vector space minimizing the functional

 (*)

Applied to (*), the method of steepest descent takes the form

where the value of is determined by minimization of the functional (*), according to the formula

If the spectrum of lies in an interval , , on the real axis, then the sequence converges to the solution at the rate of a geometric progression with quotient .

The method of steepest descent can be applied to solve an operator equation with a self-adjoint positive-definite bounded operator . If does not satisfy these conditions, the problem can be symmetrized, reduced to the problem

and then one can apply the method of steepest descent (see also Minimal discrepancy method).

#### References

 [1] L.V. Kantorovich, "On the method of steepest descent" Dokl. Akad. Nauk SSSR , 56 : 3 (1947) pp. 233–236 (In Russian) [2] L.V. Kantorovich, G.P. Akilov, "Functional analysis" , Pergamon (1982) (Translated from Russian) [3] D.K. Faddeev, V.N. Faddeeva, "Computational methods of linear algebra" , Freeman (1963) (Translated from Russian) [4] B.N. Pshenichnyi, Yu.M. Danilin, "Numerical methods in extremal problems" , MIR (1978) (Translated from Russian) [5] N.S. Bakhvalov, "Numerical methods: analysis, algebra, ordinary differential equations" , MIR (1977) (Translated from Russian)