The formula is $\partial e=\left(a{d}_e\right)b+{\sum }_{i=0}^{\infty }\left(B_i\right)/i!{\left(a{d}_e\right)}^i(b-a)$, with ∂a + 1/2 [a,a] = 0 and ∂b + 1/2 [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 $x/(e^x-1)={\sum }_{n=0}^{\infty }Bₙ/n!xⁿ$. The theorem is that ∙ this formula for ∂ on generators extends to a derivation of square zero on L[a,b,e]; ∙ the formula for ∂e is unique satisfying the first property, once given the formulae for ∂a and ∂b, along with the condition that the “flow” generated by e moves a to b in unit...