Think Eigenvector (Mona Lisa Restored) Trucker Hat
If you think in terms of eigenvalues and eigenvectors, you'll enjoy any of these gifts featuring a shear mapping of the Mona Lisa restored. Technically, the eigenvectors of a square matrix are the non-zero vectors that, after being multiplied by the matrix, remain parallel to the original vector. In this shear mapping the red arrow changes direction but the blue arrow does not. Thus, the blue arrow is an eigenvector, with eigenvalue 1 as its length is unchanged. Mathematicians and statisticians will enjoy any of these gifts featuring the Mona Lisa restored along with the caption "Think Eigenvector". For all fans of matrices!