TY - JOUR
T1 - Internal, external, and generalized symmetries
AU - Anderson, Ian M.
AU - Kamran, Niky
AU - Olver, Peter J.
PY - 1993/7
Y1 - 1993/7
N2 - Bäcklund′s theorem, which characterizes contact transformations, is generalized to give an analogous characterization of "internal symmetries" of systems of differential equations. For a wide class of systems of differential equations, we prove that every internal symmetry comes from a first order generalized symmetry and, conversely, every first order generalized symmetry satisfying certain explicit contact conditions determines an internal symmetry. We analyze the contact conditions in detail, deducing powerful necessary conditions for a system of differential equations that admit "genuine" internal symmetries, i.e., ones which do not come from classical "external" symmetries. Applications include a direct proof that both the internal symmetry group and the first order generalized symmetries of a remarkable differential equation due to Hilbert and Cartan are the noncompact real form of the exceptional simple Lie group G2.
AB - Bäcklund′s theorem, which characterizes contact transformations, is generalized to give an analogous characterization of "internal symmetries" of systems of differential equations. For a wide class of systems of differential equations, we prove that every internal symmetry comes from a first order generalized symmetry and, conversely, every first order generalized symmetry satisfying certain explicit contact conditions determines an internal symmetry. We analyze the contact conditions in detail, deducing powerful necessary conditions for a system of differential equations that admit "genuine" internal symmetries, i.e., ones which do not come from classical "external" symmetries. Applications include a direct proof that both the internal symmetry group and the first order generalized symmetries of a remarkable differential equation due to Hilbert and Cartan are the noncompact real form of the exceptional simple Lie group G2.
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U2 - 10.1006/aima.1993.1029
DO - 10.1006/aima.1993.1029
M3 - Article
AN - SCOPUS:0005195545
SN - 0001-8708
VL - 100
SP - 53
EP - 100
JO - Advances in Mathematics
JF - Advances in Mathematics
IS - 1
ER -