Tuesday, April 9, 2013

Levenberg-Marquardt Method

Levenberg-Marquardt Method

Levenberg-Marquardt is a popular alternative to the Gauss-Newton method of finding the minimum of a function F(x) that is a sum of squares of nonlinear functions,
 F(x)=1/2sum_(i=1)^m[f_i(x)]^2.
Let the Jacobian of f_i(x) be denoted J_i(x), then the Levenberg-Marquardt method searches in the direction given by the solution p to the equations
 (J_k^(T)J_k+lambda_kI)p_k=-J_k^(T)f_k,
where lambda_k are nonnegative scalars and I is the identity matrix. The method has the nice property that, for some scalar Delta related to lambda_k, the vector p_k is the solution of the constrained subproblem of minimizing ||J_kp+f_k||_2^2/2 subject to ||p||_2<=Delta (Gill et al. 1981, p. 136).
The method is used by the command FindMinimum[f, {x, x0}] when given the Method -> LevenbergMarquardt option.

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