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[9] 
Guthrie G A 1994 Proc. R. Soc. London, Ser. A 446 107

[10] 
Guthrie G A and Hickman M S 1993 J. Math. Phys. 34 193

[11] 
Krasilshchik I S and Vinogradov A M 1984 Acta Appl. Math. 2 79

[12] 
Bluman G W and Cheviakov A F 2005 J. Math. Phys. 46 123506

[13] 
Bluman G W and Kumei S 1989 Symmetries and Differential Equations (New York: SpringerVerlag)

[14] 
Lou S Y, Hu X R and Chen Y 2012 J. Phys. A: Math. Theor. 45 155209

[15] 
Hu X R, Lou S Y and Chen Y 2012 Phys. Rev. E 85 056607

[16] 
Lie S 1881 Arch. Math. 6 328

[17] 
Ovsiannikov L V 1982 Group Analysis of Differential Equations (New York: Academic)

[18] 
Olver P J 1986 Applications of Lie Groups to Differential Equations (Berlin: Springer)

[19] 
Ibragimov N H 1985 Transformation Groups Applied to Mathematical Physics (Boston, MA: Reidel)

[20] 
Chen Y and Dong Z Z 2009 Nonlinear Anal. 71 e810

[21] 
Hu X R, Chen Y and Huang F 2010 Chin. Phys. B 19 080203

[22] 
Dong Z Z, Chen Y and Lang Y H 2010 Chin. Phys. B 19 090205

[23] 
Dong Z Z, Chen Y, Kong D X and Wang Z G 2012 Chin. Ann. Math. Ser. B 33 309

[24] 
Hu X R, Chen Y and Huang F 2010 Chin. Phys. B 19 080203

[25] 
Zhang H P, Li B and Chen Y 2010 Chin. Phys. B 19 060302

[26] 
Nucci M C 1989 J. Phys. A: Math. Gen. 22 2897
