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, .