\[s_{1}(a,x)\sim 1+\frac{d_{2}}{1!2x^{2}}-\frac{c_{4}}{2!2^{2}x^{4}}-\frac{d_{6}%
}{3!2^{3}x^{6}}+\frac{c_{8}}{4!2^{4}x^{8}}+\cdots,\]