${{13}^{2}}\overset{17}{\mathop{\equiv }}\,169\overset{17}{\mathop{\equiv }}\,-1\xrightarrow{\left[ \frac{43}{2} \right]=21}{{13}^{42}}\overset{17}{\mathop{\equiv }}\,-1\xrightarrow{\times 13}{{13}^{43}}\overset{17}{\mathop{\equiv }}\,-13\Rightarrow {{13}^{43}}\overset{17}{\mathop{\equiv }}\,-13+17\overset{17}{\mathop{\equiv }}\,4$