Eikonal Blog

2010.01.22

BCS formula on computer

Filed under: mathematics — Tags: , , — sandokan65 @ 16:06

Def: e^C :\equiv e^A e^B

C = A + \int_0^1 dt \psi(e^{a}e^{t b}) B,
where a:\equiv ad(A), b:\equiv ad(B) and
\psi(z):\equiv \frac{z \ln(z)}{z-1} = 1 + \frac{w}{1\cdot 2} - \frac{w^2}{2\cdot 3} + \frac{w^3}{3\cdot 4} - \cdots for w:\equiv z-1.

Richtmyer and Greenspan obtained the 512 initial terms of that expansion, but here I present only several of them:

C = A + B + \frac12 a B + \frac1{12} a^2 B - \frac1{12} b a B + \frac1{12,096} a^4 b^2 a B - \frac1{6,048} a^3 b a^3 B - \frac1{3,780} a^3 b a b a B + ...

Not all terms are independent, due to Jacobi identities.

Sources:

  • T1267: R.D. Richtmyer and S. Greenspan: “Expansion of the Campbell-Baker-Hausdorff formula by Computer”; Communications on Pure and Applied Mathematics, Vol XVIII 107-108 (1965).
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