Euler or up/down numbers: e.g.f. sec(x) + tan(x). Also for n >= 2, half the number of alternating permutations on n letters (A001250).
A000111
Euler or up/down numbers: e.g.f. sec(x) + tan(x). Also for n >= 2, half the number of alternating permutations on n letters (A001250).
Terms
- a(0) =1a(1) =1a(2) =1a(3) =2a(4) =5a(5) =16a(6) =61a(7) =272a(8) =1385a(9) =7936a(10) =50521a(11) =353792a(12) =2702765a(13) =22368256a(14) =199360981a(15) =1903757312a(16) =19391512145a(17) =209865342976
External references
- oeis: A000111