[1]Belavin A, Polyakov A, Shvarts A and Tryupkin Yu 1975
Phys. Lett. B 59 85

[2]Yang K Y 1994 Phys. Rev. D 49 5491

[3]Fukushima M, Suganuma H and Toki H 1999 Phys. Rev. D
60 094504

[4] Cavaglia M, de Alfaro V and de Felice F 1994 Phys.
Rev. D 49 6493

[5] $'$t Hooft G 1976 Phys. Rev. Lett. 37 8

$'$t Hooft G 1976 Phys. Rev. D 14 3432

[6]Shuryak E and Verbaarschot J 1990 Nucl. Phys. B 341 1

[7]Diakonov D hep-ph/9602375

[8] Schafer T and Shuryak E 1998 Rev. Mod. Phys. 70 323

[9] Rubakov V A 1985 Nucl. Phys. B 256 509

[10]Cho Y M hep-th/9906198

[11]Faddeev L and Niemi A J hep-th/9907180

[12]Faddeev L and Niemi A J 1999 Phys. Rev. Lett. 82 1624

[13] Ford C, Tok T and Wipf A 1999 Phys. Lett. B 456 155

[14] $'$t Hooft G 1979 Nucl. Phys. B 153 141

[15] Polyakov A 1977 Nucl. Phys. B 120 429

[16]Lee K 1991 Phys. Rev. Lett. 66 553

[17] Dunne G V hep-th/9902115

[18] Nash S and Sen S 1983 Topology and Geometry of
Physicists (London: Academic)

[19] Chern S S and Simons J 1974 Ann. Math. 99 48

[20] Goursat E 1904 A Course in Mathematical Analysis
translated by Hedrick E R (New York: Dover) vol I

[21] Schouten J A 1951 Tensor Analysis for Physicists
(Oxford: Clarendon)

[22] Hopf H 1929 Math. Ann. 96 209