### How to write the code

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a sign function $\delta$

$\delta\left(x\right)$

`δ`（x） which is define on all boundary,let us consider an element(triangle) K whose vertices are numbered by global indices( $i_1^K,i^K_2,i^K_3$`i`_{1}^{K},`i`^{K}_{2},`i`^{K}_{3}) and whose edges are denoted( $F_{_{_{i^K_1i^K_2}}},F_{_{_{i^K_2i^K_3}}^{ }_2^{ }_{ }},F_{_{_{i^K_3i^K_1}}}$`F`_{iK1iK2},`F`_{iK2iK32},`F`_{iK3iK1}).Here the vertices are follows the counterclockwise direction.Then we define $\delta\left(x\right)$`δ`(`x`) as follows` `

$\delta\left(x\right)$

`δ`(`x`)=1 if $i^K_a$`i`^{K}_{a}< $i^K_b$`i`^{K}_{b},otherwise equal -1
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