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In this article we investigate the equivalence of underdetermined differential equations and differential equations with deviations of second order with respect to the pseudogroup of transformations $$ \bar x $$ = φ(x), ȳ = ȳ( $$ \bar x $$ ) = L(x) + y(x), $$ \bar z $$ = $$ \bar z $$ ( $$ \bar x $$ ) = M(x) + z(x). Our main aim is to determine such equations that admit a large...
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