User blog comment:B1mb0w/The (new) J Function/@comment-1605058-20150902182530/@comment-25337554-20150906075654

I guess

$$J(a_1,a_2,\cdots ,a_{n-1},a_n)=D(a_{n [J(a_1,a_2,\cdots ,a_{n-1})]})$$