Axel Ruhe passed away April 4, 2015. He was cross-country-skiing with friends in the Swedish mountains when after 21 km he suddenly died. He is survived by his wife Gunlaug and three children from his first marriage....

Partial least squares is a common technique for multivariate regression. The pro- cedure is recursive and in each step basis vectors are computed for the explaining variables and the solution vectors. A linear model is fitted by projection onto the span of the basis vectors. The procedure is mathematically equivalent to Golub-Kahan bidiagonalization, which is a Krylov method, and which is equiv- alent to a pair of matrix factorizations. The vectors of regression coefficients and prediction are non-linear functions of the right hand side. An algorithm for computing the Frechet derivatives of these functions is derived, based on perturbation theory for the matrix factorizations. From the Frechet derivative of the prediction vector one can compute the number of degrees of freedom, which can be used as a stopping criterion for the recursion. A few numerical examples are given.

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In this paper, we propose a method to find the best Kronecker product approximationof the blurring operator which arises in three dimensional image restoration problems. We show thatthis problem can be reduced to a well known rank-1 approximation of the scaled three dimensionalpoint spread function (PSF) array, which is much smaller. This approximation can be used as apreconditioner in solving image restoration problems with iterative methods. The comparison ofthe approximation by the new scaled PSF array and approximation by the original PSF array that is used in [J. G. Nagy and M. E. Kilmer, IEEE Trans. Image Process., 15 (2006), pp. 604–613],confirms the performance of the new proposed approximation.

An ill-posed Cauchy problem for a 3D elliptic partial differential equation with variable coefficients is considered. A well-posed quasi-boundary-value (QBV) problem is given to approximate it. Some stability estimates are given. For the numerical implementation, a large sparse system is obtained from discretizing the QBV problem using the finite difference method. A left-preconditioned generalized minimum residual method is used to solve the large system effectively. For the preconditioned system, a fast solver using the fast Fourier transform is given. Numerical results show that the method works well.

The sideways parabolic equation (SPE) is a model of the problem of determiningthe temperature on the surface of a body from the interior measurements. Mathematically it can beformulated as a noncharacteristic Cauchy problem for a parabolic partial differential equation. Thisproblem is severely ill-posed in an L2 setting. We use a preconditioned generalized minimum residualmethod (GMRES) to solve a two-dimensional SPE with variable coefficients. The preconditioner issingular and chosen in a way that allows efficient implementation using the FFT. The preconditioneris a stabilized solver for a nearby problem with constant coefficients, and it reduces the numberof iterations in the GMRES algorithm significantly. Numerical experiments are performed thatdemonstrate the performance of the proposed method.

Using the technique of semantic mirroring a graph is obtained that represents words and their translationsfrom a parallel corpus or a bilingual lexicon. The connectedness of the graph holds information about the semanticrelations of words that occur in the translations. Spectral graph theory is used to partition the graph, which leadsto a grouping of the words in different clusters. We illustrate the method using a small sample of seed words froma lexicon of Swedish and English adjectives and discuss its application to computational lexical semantics andlexicography.

Several Krylov-type procedures are introduced that generalize matrix Krylov methods for tensor computations. They are denoted minimal Krylov recursion, maximal Krylov recursion, and contracted tensor product Krylov recursion. It is proved that, for a given tensor A with multilinear rank-(p; q; r), the minimal Krylov recursion extracts the correct subspaces associated to the tensor in p+q+r number of tensor-vector-vector multiplications. An optimized minimal Krylov procedure is described that, for a given multilinear rank of an approximation, produces a better approximation than the standard minimal recursion. We further generalize the matrix Krylov decomposition to a tensor Krylov decomposition. The tensor Krylov methods are intended for the computation of low multilinear rank approximations of large and sparse tensors, but they are also useful for certain dense and structured tensors for computing their higher order singular value decompositions or obtaining starting points for the best low-rank computations of tensors. A set of numerical experiments, using real-world and synthetic data sets, illustrate some of the properties of the tensor Krylov methods.

Almost singular linear systems arise in discrete ill-posed problems. Either because ofthe intrinsic structure of the problem or because of preconditioning, the spectrum of the coefficientmatrix is often characterized by a sizable gap between a large group of numerically zero eigenvaluesand the rest of the spectrum. Correspondingly, the right-hand side has leading eigencomponentsassociated with the eigenvalues away from zero. In this paper the effect of this setting in theconvergence of the generalized minimal residual (GMRES) method is considered. It is shown thatin the initial phase of the iterative algorithm, the residual components corresponding to the largeeigenvalues are reduced in norm, and these can be monitored without extra computation. Theanalysis is supported by numerical experiments. In particular, ill-posed Cauchy problems for partialdifferential equations with variable coefficients are considered, where the preconditioner is a fast,low-rank solver for the corresponding problem with constant coefficients.

The concepts of tensors with diagonal and circulant structure are defined and aframework is developed for the analysis of such tensors. It is shown a tensor of arbitraryorder, which is circulant with respect to two particular modes, can be diagonalized inthose modes by discrete Fourier transforms. This property can be used in the efficientsolution of linear systems involving contractive products of tensors with circulantstructure. Tensors with circulant structure occur in models for image blurring withperiodic boundary conditions. It is shown that the new framework can be applied tosuch problems.