a[n] = {n,0}/(2n)!
とおく。
　a[1] = 0; a[2] = a[3] = 1/3; a[4] = 12/35; a[5] = 47/135,

　a[n] = a[n-1] + a[n-2]/{(2n-1)(2n-3)},
より
　a[n] = {1/(2n-1)!!}ｉ[Ｉ_{3/2}(-1)･Ｋ_{n+1/2}(1) - Ｋ_{3/2}(1)･Ｉ_{n+1/2}(-1) ]

ここに　Ｉ_m(z), Ｋ_m(z) は変形ベッセル函数。

Ｉ_{3/2}(z) = √(2/π) {z cosh(z) - sinh(z)} z^(-3/2)
Ｉ_{3/2}(-1) = ｉ√(2/π) (1/e)
Ｋ_{3/2}(z) = √(π/2) (1+z)exp(-z) z^(-3/2)
Ｋ_{3/2}(1) = √(π/2) (2/e),