A formula for topology/deformations and its significance
Volume 225 / 2014
Fundamenta Mathematicae 225 (2014), 229-242
MSC: Primary 55U15; Secondary 16E45, 55P35.
DOI: 10.4064/fm225-1-10
Abstract
The formula is $$\partial{e}=({\rm ad}_e)b+\sum_{i=0}^\infty{\frac{B_i}{i!}}({\rm ad}_e)^i(b-a) ,$$ with $\partial {a}+{1\over2}[a,a] =0$ and $\partial{b}+{1\over2}[b,b] =0$, where $a$, $b$ and $e$ in degrees $-1$, $-1$ and 0 are the free generators of a completed free graded Lie algebra $L[a,b,e]$. The coefficients are defined by $${x\over{e^x-1}}=\sum_{n=0}^\infty{B_n\over{}n!}x^n.$$ The theorem is that
$\bullet$ this formula for $\partial$ on generators extends to a derivation of square zero on $L[a,b,e]$;
$\bullet$ the formula for $\partial {e}$ is unique satisfying the first property, once given the formulae for $\partial{a}$ and $\partial{b}$, along with the condition that the “flow" generated by $e$ moves $a$ to $b$ in unit time. The immediate significance of this formula is that it computes the infinity cocommutative coalgebra structure on the chains of the closed interval. It may be derived and proved using the geometrical idea of flat connections and one-parameter groups or flows of gauge transformations. The deeper significance of such general DGLAs which want to combine deformation theory and rational homotopy theory is proposed as a research problem.
