## bbtauest

This function is not yet fully documented. This is a transcript of
the text-formatted help.

bbtauest Estimate accuracy of a black-box operator.
TAU=bbtauest(M) approximates the accuracy of the matrix-vector
products M*x and M'*x, when x is a unit vector.
TAU=bbtauest(M,K) uses 2*K function evaluations.
[TAU,TAUAMP]=bbtauest(M) also approximates the amplification
of previous errors, i.e. the 2-norm of M.
bbtauest will also determine suitable parameters for a normal
Matlab matrix (full or sparse). This is handy when matrices are
used as building-blocks.
The routine makes no assumptions about the values of tau and tauamp
of the operator. Therefore, it can be used to estimate the values
for an operator when a rounding error analysis is unavailable.
bbtauest offers a way to test that an operator implements the
adjoint correctly. If is usually able to point out if there is
an algorithmic mistake in the implementation of the product A'*X.
The ratio tau/tauamp should not be too large. In most cases it
should be less than 1e-10, and it should always be less than 1.
Note: estimating the rounding errors is a difficult process.
An analytical result should be used if possible.
See also , bbcompare, .

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