A note on intertwines of infinite graphs

We present a construction of two infinite graphs G1, G2 and of an infinite set F of graphs such that F is an antichain with respect to the minor relation and, for every graph G in F, both G1 and G2 are subgraphs of G but no graph obtained from G by deletion or contraction of an edge has both G1 and G2 as minors. These graphs show that the extension to infinite graphs of the intertwining conjecture of Lovász, Milgram, and Ungar fails. © 1993 Academic Press, Inc

B-series methods are exactly the affine equivariant methods

Butcher series, also called B-series, are a type of expansion, fundamental in the analysis of numerical integration. Numerical methods that can be expanded in B-series are defined in all dimensions, so they correspond to \emph{sequences of maps}---one map for each dimension. A long-standing problem has been to characterise those sequences of maps that arise from B-series. This problem is solved here: we prove that a sequence of smooth maps between vector fields on affine spaces has a B-series expansion if and only if it is \emph{affine equivariant}, meaning it respects all affine maps between affine spaces