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Can the eigenvalues of the Hessian change sign during a Newton-Raphson optimization?

Because from simple examples with 1D functions it seems as if one always converges to the nearest optimum, meaning that one stays within the turning points of the function such that the sign of the second derivative (which coincides with the eigenvalue in 1D) remains the same throughout. Does that hold in general for the Newton-Raphson method? (At least in the cases where the convergence criteria are met.)

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1 Answer 1

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It is well known that even in 1D, N-R needn't converge at all and it can hop all over the place. (Numerical Recipes has a good discussion of this behavior, as I recall.)

In more than one dimension, consider a function with a global optimum that nevertheless has regions where the Hessian is non-definite. Start the algorithm with a point in one of those regions. If you succeed eventually in finding the optimum, then at some point the signature of the Hessian must have changed. That gives a recipe for constructing counterexamples; $$f(x,y)=((x-1)^2+y^2)((x+1)^2+y^2)$$ comes to mind as being particularly simple. Anticipating factors that will appear when differentiating, I have divided its values by $4$ below.

Figure

These renderings of $f$ are by Wolfram Alpha. They show the principle of construction: each factor of $f$ is a paraboloid, one centered at $(-1,1)$ and the other at $(1,0).$ Their product creates a surface with two separated global minima (at those same points) but with a saddle in between (located at $(0,0)$).

The "saddle-shaped area" can be thought of as the region where the determinant of the Hessian is negative:

Figure 2

Thus, if any Newton-Raphson search should include points both inside and outside this region, the signs of some eigenvalues of the Hessian will change.

If we begin a Newton-Raphson search at, say, $(1/2, 0),$ it will quickly find the nearby critical point: $(1,0)$ in this case. But at $x_0 = (1/2,0)$ the Hessian is $$H[f](1/2,0) = \pmatrix{-\frac{1}{4} & 0 \\ 0 & \frac{5}{4}}$$ with a signature of $(1,1)$ (one positive and one negative eigenvalue) whereas at $x_1 = (1,0)$ the Hessian is $$H[f](1,0) = \pmatrix{2 & 0 \\ 0 & 2},$$ with a signature of $(2,0).$

The reverse change in signature can occur, too. If you begin at $(0,2),$ for instance, where the Hessian has eigenvalues $13$ and $3$ (signature $(2,0)$), then the algorithm will progress straight towards the saddlepoint at $(0,0),$ eventually changing the signature. For instance, after two steps the Hessian has eigenvalues $2.36$ and $-0.55$, with a signature of $(1,1).$


The calculations were made with this R code.

# A function to optimize.
f <- function(p) {
  x <- p[1]; y <- p[2]; ((x-1)^2 + y^2) * ((x+1)^2 + y^2) / 4
}
# The function's gradient.
f.1 <- function(p) {
  x <- p[1]; y <- p[2]; c(4*x*(x^2+y^2-1), 4*y*(x^2+y^2+1)) / 4
}
# The function's Hessian.
f.2 <- function(p) {
  x <- p[1]; y <- p[2]; matrix(c(4*(3*x^2+y^2-1), 8*x*y, 8*x*y, 4*(x^2+3*y^2+1)), 2) / 4
}
#
# Quick-and-dirty Newton-Raphson iteration with diagnostic printing.
#
NR <- function(f, df, x.0, xtol=1e-8, ftol=1e-8, itermax=100) {
  x <- x.0
  y <- f(x)   # Vector
  dy <- df(x) # Matrix
  for (iter in 1:itermax) {
    cat(paste("Iteration", iter), ": ")
    cat(eigen(dy, only.values=TRUE)$values, "\n")
    dx <- solve(dy, -y)
    if (sum(dx^2) <= xtol^2) break
    x <- x + dx
    y <- f(x)
    dy <- df(x)
    if (sum(y^2) <= ftol^2) break
  }
  cat(paste("Iteration", iter), ": ")
  cat(eigen(dy, only.values=TRUE)$values, "\n")
  list(x = x, y = y, niter = iter+1)
}
#
# Examples.
#
NR(f.1, f.2, c(1/2, 0), itermax=12)
NR(f.1, f.2, c(0, 2), itermax=12)
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