419730685952domain: NAppears in sequencesNumber of alternating permutations of order n.at n=17A001250E.g.f.: -log(cos(x)*cos(x)) (even powers only).at n=9A012509Expansion of sec(x)^2+sech(x)^2 in powers of x^4.at n=4A012853Twice the Euler or up/down numbers A000111.at n=17A260786Expansion of e.g.f.: sec(x) + 2*tan(x).at n=17A309845