A set partition technique that is useful for identifying wires in cables can be recast in the language of 0–1 matrices, thereby resolving an open problem stated by R. L. Graham in Volume 1 of this journal. The proof involves a construction of 0–1 matrices having row and column sums without gaps. A long cable contains n indistinguishable wires. Two people, one at each end, want to label the wire...

The relationship between Bach’s tensor and the four dimensional conformal anomaly is clarified, as well as its rôle as an obstruction for a perturbative reconstruction of the hologram (a Dirichlet problem). Some considerations on the obstruction in higher dimensions are included and, in particular, a candidate for the Fefferman-Graham obstruction in six dimensional space is constructed.