Nonlocal double phase Neumann and Robin problem with variable $s(\cdot,\cdot)-$order

Abstract: In this paper, we develop some properties of the $a_{x,y}(\cdot)$-Neumann derivative for the nonlocal $s(\cdot,\cdot)$-order operator in fractional Musielak-Sobolev spaces with variable $s(\cdot,\cdot)-$order. Therefore we prove the basic proprieties of the correspondent function spaces. In the second part of this paper, by means of Ekeland's variational principal and direct variational approach, we prove the existence of weak solutions to the following double phase Neumann and Robin problem with variable $s(\cdot,\cdot)-$order: $$\left{\begin{array} (-\Delta)^{s_1(x,\cdot)}{a^1{(x,\cdot)}} u+(-\Delta)^{s_2(x,\cdot)}{a^2{(x,\cdot)}} u +\widehat{a}^1_x(|u|)u+\widehat{a}^2_x(|u|)u &= \lambda f(x,u) \quad {\rm in\ } \Omega, \ \mathcal{N}^{s_1(x,\cdot)}{a^1(x,\cdot)}u+\mathcal{N}^{s_2(x,\cdot)}{a^2(x,\cdot)}u+\beta(x)\left( \widehat{a}^1_x(|u|)u+\widehat{a}^2_x(|u|)u \right) &= 0 \quad {\rm in\ } \mathbb{R}^N\setminus \Omega, \end{array} \right. $$ where $(-\Delta)^{s_i(x,\cdot)}{a^i{(x,\cdot)}}$ and $\mathcal{N}^{s_i(x,\cdot)}_{a^i(x,\cdot)}$ denote the variable $s_i(\cdot,\cdot)$-order fractional Laplace operator and the nonlocal normal $a_i(\cdot,\cdot)$-derivative of $s_i(\cdot,\cdot)$-order, respectively.