    April  2019, 39(4): 1957-1974. doi: 10.3934/dcds.2019082

## Binary differential equations with symmetries

 1 Instituto de Ciências Matemáticas e de Computação, Universidade de São Paulo-Campus de São Carlos, Caixa Postal 668, 13560-970 São Carlos SP, Brazil 2 Departamento de Matemática e Estatística, Universidade Federal S. J. del Rei, P. Frei Orlando, 170, Centro, S. J. del Rei, MG, 36307-352, Brazil

Received  February 2018 Revised  September 2018 Published  January 2019

Fund Project: This study was financed in part by Coordenação de Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) - Finance Code 001. Grant number 8474758/D for P.T. and CAPES/FCT grant 88887.125430/2016-00 for M.M

This paper introduces the study of occurrence of symmetries in binary differential equations (BDEs). These are implicit differential equations given by the zeros of a quadratic 1-form, $a(x,y)dy^2 + b(x,y)dxdy + c(x,y)dx^2 = 0,$ for $a, b, c$ smooth real functions defined on an open set of $\mathbb{R}^2$. Generically, solutions of a BDE are given as leaves of a pair of foliations, and the action of a symmetry must depend not only whether it preserves or inverts the plane orientation, but also whether it preserves or interchanges the foliations. The first main result reveals this dependence, which is given algebraically by a formula relating three group homomorphisms defined on the symmetry group of the BDE. The second main result adapts methods from invariant theory of compact Lie groups to obtain an algorithm to compute general expressions of equivariant quadratic 1-forms under each compact subgroup of the orthogonal group ${{\bf{O}}(2)}$.

Citation: Miriam Manoel, Patrícia Tempesta. Binary differential equations with symmetries. Discrete & Continuous Dynamical Systems - A, 2019, 39 (4) : 1957-1974. doi: 10.3934/dcds.2019082
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##### References:
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General forms of equivariant quadratic differential forms on the plane under closed subgroups of O$(2)$
 $\Gamma[\ker \eta]$ $\ker \lambda$ General form ${\bf{SO}}(2)$ ${\bf{SO}}(2)$ $a = p_1 + (y^2-x^2)p_2 + 2xyp_3;$ $b = 2xyp_2 + (x^2-y^2)p_3;$ $c = p_1 + (x^2-y^2)p_2 - 2xyp_3,$ $p_i \in \mathcal{P}({\bf{SO}}(2)), i = 1,2,3.$ ${\bf{O}}(2)$ ${\bf{SO}}(2)$ $a = p_1 + (y^2-x^2)p_2; \ b = 2xyp_2;$ $c = p_1 + (x^2-y^2)p_2, \ p_i \in \mathcal{P}({\bf{O}}(2)), i = 1,2.$ ${\bf{O}}(2)[{\bf{SO}}(2)]$ ${\bf{O}}(2)$ $a = 2xyp;$ $b = (x^2-y^2)p; $$c = -2xyp, p \in \mathcal{P}({\bf{O}}(2)). {\bf Z}_n, \\ n \geq 3 {\bf Z}_n a = p_1 + (y^2-x^2)p_2 + 2xyp_3 -A_1p_4 -A_2p_5; b = 2xyp_2 + (x^2-y^2)p_3 + A_1p_5 -A_2p_4; c = p_1 + (x^2-y^2)p_2 -2xyp_3 +A_1p_4 + A_2p_5, p_i \in \mathcal{P}({\bf Z}_n ), i = 1,..., 5. {\bf Z}_n[{\bf Z}_{n/2}], n \geq 4 even {\bf Z}_{n/2} a = -A_3p_1-A_4p_2+A_5p_3-A_7p_4+A_8p_5+A_6p_6; b = -A_4p_1 + A_3p_2 + A_8p_4 + A_7p_5;$$ c = A_3p_1 + A_4p_2 + A_5p_3 +A_7p_4 - A_8p_5 + A_6p_6, $$p_i \in \mathcal{P}({\bf Z}_n), i = 1, ..., 6. {\bf{D}}_n,$$ n \geq 3$ ${\bf Z}_n$ $a = p_1 + (y^2-x^2)p_2 - A_1p_3;$ $b = 2xyp_2 - A_2p_3; $$c = p_1 + (x^2-y^2)p_2 + A_1p_3,$$ p_i \in \mathcal{P}({\bf{D}}_n), i = 1,2,3.$ ${\bf{D}}_n[{\bf Z}_n],$ $n \geq 3$ ${\bf{D}}_n$ $a = 2xyp_1 -A_2p_2 +A_9p_3;$ $b = (x^2-y^2)p_1 + A_1p_2; $$c = -2xyp_1 + A_2p_2 +A_9p_3,$$ \ p_i \in \mathcal{P}({\bf{D}}_n), i = 1, 2, 3.$ ${\bf{D}}_n[{\bf{D}}_{n/2}(\kappa_x)], $$n \geq 4 even {\bf{D}}_{n/2}(\kappa_y)] a = -A_3p_1 +A_5p_2 -A_7p_3; b = -A_4p_1 +A_8p_3; c = A_3p_1 +A_5p_2 + A_7p_3,$$ p_i \in \mathcal{P}({\bf{D}}_n), i = 1, 2, 3.$ ${\bf Z}_2$ ${\bf 1}$ $a = p_1; \ b = yp_2; \ c = p_3, $$p_i \in \mathcal{P}({\bf Z}_2), i = 1, 2,3. {\bf Z}_2[{\bf 1}] {\bf Z}_2 a = y p_1; \ b = p_2; \ c = yp_3,$$ p_i \in \mathcal{P}({\bf Z}_2), i = 1, 2, 3.$ ${\bf Z}_2 \times {\bf Z}_2$ ${\bf Z}_2(-I)$ $a = p_1; \ b = xyp_2; \ c = p_3, $$p_i \in \mathcal{P}({\bf Z}_2 \times {\bf Z}_2), i = 1, 2, 3. {\bf Z}_2 \times {\bf Z}_2[{\bf Z}_2(-I)] {\bf Z}_2 \times {\bf Z}_2 a = xyp_1; \ b = p_2; \ c = xyp_3,$$ p_i \in \mathcal{P}({\bf Z}_2 \times {\bf Z}_2), i = 1, 2, 3.$ ${\bf Z}_2 \times {\bf Z}_2[{\bf Z}_2(\kappa_x)]$ ${\bf Z}_2(\kappa_y)$ $a = xp_1; \ b = yp_2; \ c = xp_3, $$p_i \in \mathcal{P}({\bf Z}_2 \times {\bf Z}_2), i = 1, 2, 3. A_1 = \hbox{Re}(z^{n-2}), \ A_2 = \hbox{Im}(z^{n-2}), \ A_3 = \hbox{Re}(z^{n/2-2}), \ A_4 = \hbox{Im}(z^{n/2-2}), \ A_5 = \hbox{Re}(z^{n/2}), \ A_6 = \hbox{Im}(z^{n/2}), \ A_7 = \hbox{Re}(z^{n/2+2}), \ A_8 = \hbox{Im}(z^{n/2+2}), \ A_9 = \hbox{Im}(z^n).  \Gamma[\ker \eta] \ker \lambda General form {\bf{SO}}(2) {\bf{SO}}(2) a = p_1 + (y^2-x^2)p_2 + 2xyp_3; b = 2xyp_2 + (x^2-y^2)p_3; c = p_1 + (x^2-y^2)p_2 - 2xyp_3, p_i \in \mathcal{P}({\bf{SO}}(2)), i = 1,2,3. {\bf{O}}(2) {\bf{SO}}(2) a = p_1 + (y^2-x^2)p_2; \ b = 2xyp_2; c = p_1 + (x^2-y^2)p_2, \ p_i \in \mathcal{P}({\bf{O}}(2)), i = 1,2. {\bf{O}}(2)[{\bf{SO}}(2)] {\bf{O}}(2) a = 2xyp; b = (x^2-y^2)p;$$ c = -2xyp, p \in \mathcal{P}({\bf{O}}(2)).$ ${\bf Z}_n, \\ n \geq 3$ ${\bf Z}_n$ $a = p_1 + (y^2-x^2)p_2 + 2xyp_3 -A_1p_4 -A_2p_5;$ $b = 2xyp_2 + (x^2-y^2)p_3 + A_1p_5 -A_2p_4;$ $c = p_1 + (x^2-y^2)p_2 -2xyp_3 +A_1p_4 + A_2p_5,$ $p_i \in \mathcal{P}({\bf Z}_n ), i = 1,..., 5.$ ${\bf Z}_n[{\bf Z}_{n/2}],$ $n \geq 4$ even ${\bf Z}_{n/2}$ $a = -A_3p_1-A_4p_2+A_5p_3-A_7p_4+A_8p_5+A_6p_6;$ $b = -A_4p_1 + A_3p_2 + A_8p_4 + A_7p_5; $$c = A_3p_1 + A_4p_2 + A_5p_3 +A_7p_4 - A_8p_5 + A_6p_6,$$ p_i \in \mathcal{P}({\bf Z}_n), i = 1, ..., 6.$ ${\bf{D}}_n, $$n \geq 3 {\bf Z}_n a = p_1 + (y^2-x^2)p_2 - A_1p_3; b = 2xyp_2 - A_2p_3;$$ c = p_1 + (x^2-y^2)p_2 + A_1p_3, $$p_i \in \mathcal{P}({\bf{D}}_n), i = 1,2,3. {\bf{D}}_n[{\bf Z}_n], n \geq 3 {\bf{D}}_n a = 2xyp_1 -A_2p_2 +A_9p_3; b = (x^2-y^2)p_1 + A_1p_2;$$ c = -2xyp_1 + A_2p_2 +A_9p_3, $$\ p_i \in \mathcal{P}({\bf{D}}_n), i = 1, 2, 3. {\bf{D}}_n[{\bf{D}}_{n/2}(\kappa_x)],$$ n \geq 4$ even ${\bf{D}}_{n/2}(\kappa_y)]$ $a = -A_3p_1 +A_5p_2 -A_7p_3;$ $b = -A_4p_1 +A_8p_3;$ $c = A_3p_1 +A_5p_2 + A_7p_3, $$p_i \in \mathcal{P}({\bf{D}}_n), i = 1, 2, 3. {\bf Z}_2 {\bf 1} a = p_1; \ b = yp_2; \ c = p_3,$$ p_i \in \mathcal{P}({\bf Z}_2), i = 1, 2,3.$ ${\bf Z}_2[{\bf 1}]$ ${\bf Z}_2$ $a = y p_1; \ b = p_2; \ c = yp_3, $$p_i \in \mathcal{P}({\bf Z}_2), i = 1, 2, 3. {\bf Z}_2 \times {\bf Z}_2 {\bf Z}_2(-I) a = p_1; \ b = xyp_2; \ c = p_3,$$ p_i \in \mathcal{P}({\bf Z}_2 \times {\bf Z}_2), i = 1, 2, 3.$ ${\bf Z}_2 \times {\bf Z}_2[{\bf Z}_2(-I)]$ ${\bf Z}_2 \times {\bf Z}_2$ $a = xyp_1; \ b = p_2; \ c = xyp_3, $$p_i \in \mathcal{P}({\bf Z}_2 \times {\bf Z}_2), i = 1, 2, 3. {\bf Z}_2 \times {\bf Z}_2[{\bf Z}_2(\kappa_x)] {\bf Z}_2(\kappa_y) a = xp_1; \ b = yp_2; \ c = xp_3,$$ p_i \in \mathcal{P}({\bf Z}_2 \times {\bf Z}_2), i = 1, 2, 3.$ $A_1 = \hbox{Re}(z^{n-2}), \ A_2 = \hbox{Im}(z^{n-2}), \ A_3 = \hbox{Re}(z^{n/2-2}), \ A_4 = \hbox{Im}(z^{n/2-2}), \ A_5 =$ $\hbox{Re}(z^{n/2}), \ A_6 = \hbox{Im}(z^{n/2}), \ A_7 = \hbox{Re}(z^{n/2+2}), \ A_8 = \hbox{Im}(z^{n/2+2}), \ A_9 = \hbox{Im}(z^n).$
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