In differential geometry and gauge theory, the Nahm equations are a system of ordinary differential equations introduced by Werner Nahm in the context of the Nahm transform – an alternative to Ward's twistor construction of monopoles. The Nahm equations are formally analogous to the algebraic equations in the ADHM construction of instantons, where finite order matrices are replaced by differential operators.

Deep study of the Nahm equations was carried out by Nigel Hitchin and Simon Donaldson. Conceptually, the equations arise in the process of infinite-dimensional hyperkähler reduction. They can also be viewed as a dimensional reduction of the anti-self-dual Yang-Mills equations (Donaldson 1984). Among their many applications we can mention: Hitchin's construction of monopoles, where this approach is critical for establishing nonsingularity of monopole solutions; Donaldson's description of the moduli space of monopoles; and the existence of hyperkähler structure on coadjoint orbits of complex semisimple Lie groups, proved by (Kronheimer 1990), (Biquard 1996), and (Kovalev 1996).

Equations

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Let   be three matrix-valued meromorphic functions of a complex variable  . The Nahm equations are a system of matrix differential equations

 

together with certain analyticity properties, reality conditions, and boundary conditions. The three equations can be written concisely using the Levi-Civita symbol, in the form

 

More generally, instead of considering   by   matrices, one can consider Nahm's equations with values in a Lie algebra  .

Additional conditions

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The variable   is restricted to the open interval  , and the following conditions are imposed:

  1.  
  2.  
  3.   can be continued to a meromorphic function of   in a neighborhood of the closed interval  , analytic outside of   and  , and with simple poles at   and  ; and
  4. At the poles, the residues of   form an irreducible representation of the group SU(2).

Nahm–Hitchin description of monopoles

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There is a natural equivalence between

  1. the monopoles of charge   for the group  , modulo gauge transformations, and
  2. the solutions of Nahm equations satisfying the additional conditions above, modulo the simultaneous conjugation of   by the group  .

Lax representation

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The Nahm equations can be written in the Lax form as follows. Set

 

then the system of Nahm equations is equivalent to the Lax equation

 

As an immediate corollary, we obtain that the spectrum of the matrix   does not depend on  . Therefore, the characteristic equation

 

which determines the so-called spectral curve in the twistor space   is invariant under the flow in  .

See also

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References

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  • Nahm, W. (1981). "All self-dual multimonopoles for arbitrary gauge groups". CERN, Preprint TH. 3172.
  • Hitchin, Nigel (1983). "On the construction of monopoles". Communications in Mathematical Physics. 89 (2): 145–190. Bibcode:1983CMaPh..89..145H. doi:10.1007/BF01211826. S2CID 120823242.
  • Donaldson, Simon (1984). "Nahm's equations and the classification of monopoles". Communications in Mathematical Physics. 96 (3): 387–407. Bibcode:1984CMaPh..96..387D. doi:10.1007/BF01214583. S2CID 119959346.
  • Atiyah, Michael; Hitchin, N. J. (1988). The geometry and dynamics of magnetic monopoles. M. B. Porter Lectures. Princeton, NJ: Princeton University Press. ISBN 0-691-08480-7.
  • Kronheimer, Peter B. (1990). "A hyper-Kählerian structure on coadjoint orbits of a semisimple complex group". Journal of the London Mathematical Society. 42 (2): 193–208. doi:10.1112/jlms/s2-42.2.193.
  • Kovalev, A. G. (1996). "Nahm's equations and complex adjoint orbits". Quart. J. Math. Oxford. 47 (185): 41–58. doi:10.1093/qmath/47.1.41.
  • Biquard, Olivier (1996). "Sur les équations de Nahm et la structure de Poisson des algèbres de Lie semi-simples complexes" [Nahm equations and Poisson structure of complex semisimple Lie algebras]. Math. Ann. 304 (2): 253–276. doi:10.1007/BF01446293. S2CID 73680531.
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